My neighbour asked me to go for ________ walk, but I don't think I've got ________ energy. [ ]

A. a; /

B. the; the

C. /; the

D. a; the

材料的堆积密度是指散粒状材料在（）下单位体积的质量。

A.绝对密实状态

B.自然状态

C.自然堆积状态

D.绝对密实状态近似值