下列行为属于先天性行为的是（    ）

A．狗拉雪撬

B．马拉车

C．蚕吐丝

D．猴子剥花生

We are organizing a lot of activities to celebrate the _______ birthday of the Communist Party of China these months.

A．nineteen

B．ninety

C．nineteenth

D．ninetieth