TOEFL Reading: ETS-TOEFL阅读机经 - 45FTU19J55P_DVM6L$

According to paragraph 2, why do astronomers believe that the nebular cloud formed a disk around the protosun? A. They can still see some debris from the disk. B. They have observed that disks have formed around other stars. C. They know that any rotating cloud of gas tends to contract into a disk shape. D. They have conducted experiments with gravity that have confirmed their belief.