MOOCs, short for "massive open online courses," mark an important, possibly revolutionary, development in education. These courses are on line, free of charge, and open to anyone in the world who has a laptop and an Internet connection. Moreover, they are mainly offered by great universities like Stanford, Berkeley, Harvard and Columbia. The courses are arranged according to how difficult they are, enabling students to progress from beginners to the advanced. The courses cover not only a broad range of science subjects such as math and computer science, but also courses in social sciences and humanities. Though MOOCs are not offered for credit and degree, many students enroll  in the courses for real skills or knowledge which they can put to some practical use. The format  seems better than the traditional school class. The average quality of the lecturers is much higher. Besides, students can learn back and forward-that is, they can go at their own learning speed, which they can't do in a live lecture. And, more importantly, they don't have to travel anywhere to attend an online lecture. There is a problem of asking questions of the lecturer in a class of ten thousand students, but some MOOCs have solved it by allowing students to post questions on line for a vote, and only the most popular questions are put to the lecturer. In a knowledge age, lifelong learning is not _ to a traditional classroom. Students taking MOOCS are usually very clever, have work experience, and in many cases, have already developed a set of practical skills. Moreover, they also offer unique international perspectives  that would be the envy of any school classroom. MOOCs seem to have an advantage over traditional school classes in that   _  .
there is a problem of asking questions in traditional classes students enrolling in MOOCs can get credit and degree easily students can learn at their own study pace students can travel to many places when taking MOOCs
Question: MOOCs, short for "massive open online courses," mark an important, possibly revolutionary, development in education. These courses are on line, free of charge, and open to anyone in the world who has a laptop and an Internet connection. Moreover, they are mainly offered by great universities like Stanford, Berkeley, Harvard and Columbia. The courses are arranged according to how difficult they are, enabling students to progress from beginners to the advanced. The courses cover not only a broad range of science subjects such as math and computer science, but also courses in social sciences and humanities. Though MOOCs are not offered for credit and degree, many students enroll  in the courses for real skills or knowledge which they can put to some practical use. The format  seems better than the traditional school class. The average quality of the lecturers is much higher. Besides, students can learn back and forward-that is, they can go at their own learning speed, which they can't do in a live lecture. And, more importantly, they don't have to travel anywhere to attend an online lecture. There is a problem of asking questions of the lecturer in a class of ten thousand students, but some MOOCs have solved it by allowing students to post questions on line for a vote, and only the most popular questions are put to the lecturer. In a knowledge age, lifelong learning is not _ to a traditional classroom. Students taking MOOCS are usually very clever, have work experience, and in many cases, have already developed a set of practical skills. Moreover, they also offer unique international perspectives  that would be the envy of any school classroom. MOOCs seem to have an advantage over traditional school classes in that   _  .
A. there is a problem of asking questions in traditional classes
B. students enrolling in MOOCs can get credit and degree easily
C. students can learn at their own study pace
D. students can travel to many places when taking MOOCs
Answer: