颗粒群的平均粒度表示方法有哪些？
参考答案：算术平均粒径；几何平均粒径；调和平均粒径，算术平均粒径＞几何平均粒径＞调和平均粒径。
艾滋病的主要传播途径有
A．性接触
B．蚊虫叮咬
C．器官移植
D．母婴垂直传播
E．输入血液及血制品
"We’re not bringing in millions of dollars," says a director of development. "But we want to make sure the demand is there before we act to the project. "()
A. of
B. off
C. on
D. for
