堆密度是指()。
A．真密度
B．颗粒密度
C．松密度
D．绝干密度
E．比密度
参考答案：C
解析：[知识点] 粉体学简介
钟表的轴心到分针针端的长为4cm，那么经过40分钟，分针针端转过的弧长是______ cm（用π表示）．
— How was your school trip?
— . [ ]
A. it's pretty good
B. OK
C. It was great
D. That sounds interesting
