催化剂的骨架密度大于颗粒密度，催化剂的堆积密度小于颗粒密度。
参考答案：对
35．—Sorry for having kept you waiting ．But for the traffic jam,I________20 minutes earlier．—Never mind．I haven’t been waiting that long．
A．arrived
B．would arrive
C．would have arrived
D．had arrived
体育科学研究工作的特点有哪些？
