人们相信军队已经跨过边境线。
A. It is believed that the troops have crossed the border.
B. It is believed that the troops had crossed the border.
C. People believed that the troops have crossed the border.
D. It is believed that the troops have already crossed the border.
据说我们的近海海域储藏有大量的石油。
A. It is said that we have plenty of oil off our coast.
B. People said that we have plenty of oil off our coast.
C. It is said that there is plenty of oil off our coast.
D. It is said that there is a plenty of oil off our coast.
胚胎的发育三个阶段为____、____、____。