一氧化氮（NO）是人体中枢神经系统的信号因子，其中氮元素的化合价是（　　）

A．+1

B．+2

C．+3

D．+4

The ______ sound of movements in the kitchen suggested that the children were secretly helping themselves to the chocolate cake.

A．faint

B．vague

C．slender

D．dim