一道GWD的题 to be 和 at 的用法，谢谢大牛们！

joyous314

215. (30128-!-item-!-188;#058&004836)     (GWD-31-Q28)
Rock samples taken from the remainsof an asteroid about twice the size of the 6-mile-wide asteroid that eradicatedthe dinosaurs has been dated to be 3.47 billion years old and thus isevidence of the earliest known asteroid impact on Earth.
A. has been dated to be 3.47 billionyears old and thus is
B. has been dated at 3.47 billionyears old and thus
C. have been dated to be 3.47billion years old and thus are
D. have been dated as being 3.47billion years old and thus
E. have been dated at 3.47 billionyears old and thus are
答案E 想请教一下C中的to be 3.47和E中的 at 3.47的区别在哪里呢？

joyous314

哦！谢谢

孩子气0103

at加old; to be加ago
eg. have been dated at 3.47 billionyears old and thus are
have been dated to be 3.47 billionyears ago and thus are