一道英语难题

我儿子做了一道题，是这样的。Wenchuan earthquake is believed______ by the movement of the surface of the earth。 A. to be caused B. to have been caused C. to cause D. having been caused
答案给的是 B。可是我儿子认为应该选A。因为他的高中二年级下册，unit12 的课文中就有这样一句话，The thing is believed to be caused by a sea monster. 而且他的配套资料上有这么一个例句The fire is believed to be caused by carelessness.就这一问题请水老师给予解答。谢谢！急！急！急！