By the 1920s, Howard Carter ____ an explorer.

A．has become

B．had become

C．became

D．would become

粗直径钢筋锥螺纹连接的优点有()。

A.对中性好 

B.可连接异径钢筋 

C.锥螺纹套筒局部加粗，连接断面连接强度高于母材 

D.现场套丝质量可靠 

E.可连接水平、垂直、斜向钢筋