He did not regret saying what he did but felt that he         it differently. [ ]

A. could express  

B. would express    

C. could have expressed      

D. must have expressed

1微米为千分之一毫米，1纳米又等于（）微米，相当于头发丝的十万分之一。

A、百分之一

B、千分之一

C、万分之一

D、十万分之一