By the time he was twelve, Edison _______ to make a living by himself.

A．would begin

B．has begun

C．had begun

D．began

沙门菌的抗原容易发生变异，变异的种类有（）。

A.H-O变异

B.S-R变异

C.V-w变异

D.以上都是

E.以上均不对