在每一个生态系统中，影响该生态系统特性的是 [ ]

A．植物种类的多少

B．动物种类的多少

C．人类干预的程度

D．各种环境因素

When he saw we were upset, he began to ____ us for hours with his stories and jokes.

A．encourage

B．educate

C．entertain

D．astonish