Mr. Brown _____ go to bed early, but now he ____going to bed early．

A．used to, used to

B．used to, is used to

C．is used to, sued to

D．is used to, is used to

快中子是指中子能量大于（）eV的中子。

A.5×10⁴

B.5×10⁵

C.5×10⁶

D.5×10⁷