The air bag is a piece of safety equipment in cars. It was first designed by John W. Hetrick in 1952. Like many inventions, he came up with the idea as a result of an event that had occurred in his life. He says: "In the spring of 1952, my wife, my seven-year-old daughter, Joan, and I were out for a Sunday drive in our 1948 Chrysler Windsor. About three miles outside Newport we were watching for deer jumping across the road. Suddenly, there was a large rock in our path. I hit the brakes and we went into a ditch . "As I applied the brakes, both my wife and I threw our hands up to keep our daughter from hitting the car. There was soft mud in the ditch, so the car wasn't damaged, and no one was hurt." "During the ride home I couldn't stop thinking about the accident. I asked myself,'Why couldn't some object come out to stop you from striking the inside of the car?'As soon as I got home that night I drew some sketches . Each evening for the following two weeks, I'd add or reduce something from the sketches." Hetrick applied an event he had observed while in the Navy to the design of the air bag. He was repairing a torpedo  which had a cloth covering. When the compressed  air that was in the torpedo was let out, the covering was suddenly filled with air and was shot to the ceiling. With this knowledge, he developed his design until he was able to obtain a patent on the invention on August 5, 1952. The idea was similar to the air bag designs of today. Compressed air is stored in a container and when a traffic accident occurs and the car slows down at a rapid enough rate, the air will be released into the air bag. The idea was _ , but Hetrick's air bag never would have functioned properly. It was really a breakthrough, but it would require years and years of designing and testing by some top car designers before it could be used. The text is mainly about   _  .
the great inventor Hetrick The invention of air bags a terrible car accident road safety in the 1950s
Question: The air bag is a piece of safety equipment in cars. It was first designed by John W. Hetrick in 1952. Like many inventions, he came up with the idea as a result of an event that had occurred in his life. He says: "In the spring of 1952, my wife, my seven-year-old daughter, Joan, and I were out for a Sunday drive in our 1948 Chrysler Windsor. About three miles outside Newport we were watching for deer jumping across the road. Suddenly, there was a large rock in our path. I hit the brakes and we went into a ditch . "As I applied the brakes, both my wife and I threw our hands up to keep our daughter from hitting the car. There was soft mud in the ditch, so the car wasn't damaged, and no one was hurt." "During the ride home I couldn't stop thinking about the accident. I asked myself,'Why couldn't some object come out to stop you from striking the inside of the car?'As soon as I got home that night I drew some sketches . Each evening for the following two weeks, I'd add or reduce something from the sketches." Hetrick applied an event he had observed while in the Navy to the design of the air bag. He was repairing a torpedo  which had a cloth covering. When the compressed  air that was in the torpedo was let out, the covering was suddenly filled with air and was shot to the ceiling. With this knowledge, he developed his design until he was able to obtain a patent on the invention on August 5, 1952. The idea was similar to the air bag designs of today. Compressed air is stored in a container and when a traffic accident occurs and the car slows down at a rapid enough rate, the air will be released into the air bag. The idea was _ , but Hetrick's air bag never would have functioned properly. It was really a breakthrough, but it would require years and years of designing and testing by some top car designers before it could be used. The text is mainly about   _  .
A. the great inventor Hetrick
B. The invention of air bags
C. a terrible car accident
D. road safety in the 1950s
Answer: