Andy was crying in his room just now，but he wouldn’t tell me_______ made him so sad.

A．why it was that

B．why was it that

C．what it was that

D．what was it that

心室肌动作电位平台期的形成是由于（）

A.Na+内流，Cl-外流

B.Na+内流，K+外流

C.Na+内流，Cl-内流

D.Ca++内流，K+外流

E.K+内流，Ca++外流