以下表现可有助于绞窄性肠梗阻诊断的是（）

A.腹部阵发性绞痛

B.呕吐出现早而频繁

C.全腹胀

D.肠鸣音亢进

E.腹腔穿刺抽出血性液体

Kevin was not good at Chinese at first. After two months in China, _____, he made great progress.

A．while

B．but

C．however

D．otherwise