The journey two naval   officers made some time ago to the very deepest point on the earth makes us realize how much of the world still remains to be explored. The two men went down seven miles to the bottom of the Pacific Ocean inside a small steel ball called a "bathyscaphe"   to find out if there are any ocean currents or sings of life.  It was necessary to set out early, so that the "bathyscaphe" would come to the surface in daylight, and so be easily found by the mother ship which would be waiting for it. The drivers began preparations at dawn and soon afterwards, when all was ready, the steel ball disappeared under the surface of the water.  In time, the temperature dropped to freezing-point and the men trembled   inside the ball. They kept in touch with the mother ship by telephone describing how they felt. Then, at a depth of 3,000 feet, the men were surprised by a sudden, loud noise, even the smallest hole in the ball would have meant instant  death. Luckily, though, it was only one of the outer windows that had broken. Soon afterwards, the "bathyscaphe" touched the soft ocean floor raising a big cloud of "dust" made up of different kinds of small, dead sea animals. Here, powerful lights lit up the dark water and the men were surprised to see fish swimming just above them quite uncontrolled by the very large water-pressure. But they did not dare to leave the lights on for long, as the heat from them made the water boil. Quite unexpectedly, the telephone began working again and the weak but clear voices of the officers wre heard on the mother ship. After a stay of thirty minutes the men began their journey up, arriving three hours later,  _ . The officers started their journey at dawn   _  .
because they could return in 24 hours because the sea was calm then when the sun was not strong so that they could return before dark
Question: The journey two naval   officers made some time ago to the very deepest point on the earth makes us realize how much of the world still remains to be explored. The two men went down seven miles to the bottom of the Pacific Ocean inside a small steel ball called a "bathyscaphe"   to find out if there are any ocean currents or sings of life.  It was necessary to set out early, so that the "bathyscaphe" would come to the surface in daylight, and so be easily found by the mother ship which would be waiting for it. The drivers began preparations at dawn and soon afterwards, when all was ready, the steel ball disappeared under the surface of the water.  In time, the temperature dropped to freezing-point and the men trembled   inside the ball. They kept in touch with the mother ship by telephone describing how they felt. Then, at a depth of 3,000 feet, the men were surprised by a sudden, loud noise, even the smallest hole in the ball would have meant instant  death. Luckily, though, it was only one of the outer windows that had broken. Soon afterwards, the "bathyscaphe" touched the soft ocean floor raising a big cloud of "dust" made up of different kinds of small, dead sea animals. Here, powerful lights lit up the dark water and the men were surprised to see fish swimming just above them quite uncontrolled by the very large water-pressure. But they did not dare to leave the lights on for long, as the heat from them made the water boil. Quite unexpectedly, the telephone began working again and the weak but clear voices of the officers wre heard on the mother ship. After a stay of thirty minutes the men began their journey up, arriving three hours later,  _ . The officers started their journey at dawn   _  .
A. because they could return in 24 hours
B. because the sea was calm then
C. when the sun was not strong
D. so that they could return before dark
Answer: