问题
选择题
. The reason for all the changes being made _______to us yet.
A．has not explained
B．has not been explained
C．did not explained
D．were not explained
答案
答案：B
考查时态。根据题中时间状语yet可知用现在完成时态，根据语境可知用被动形式。句意：正在作所有改变的原因还没有向我们解释。