______ that a society like the National Society for the Prevention of Cruelty to Children is still needed in a civilized country

A．How does it come

B．How it comes

C．How did it come about

D．How did it come

¹³N-NH₃进入心肌细胞的机制，下列哪项是正确的()

A．Na⁺-K⁺-ATP酶转运

B．心肌细胞主动摄取

C．自由扩散

D．心肌细胞胞饮

E．Ca²⁺-通道转运