一I heard that you got an A in the chemistry exam．Well done!

一Thanks，I know that you     a good job as well．

A．shouId have done

B．need have done

C．must have done

D．could have done

中隔导联V3和V4的主波应为()

A．正波

B．负波

C．正负相等

D．正相波为主

E．等电位