He ______ the club last year. [ ]
A. took part in
B. joined
C. join
D. joins
答案:B
胃癌的主要症状是()
A.上腹部疼痛
B.食欲减退
C.恶心、呕吐
D.呕血、黑便
E.低热、体重减轻
应变正比于应力,可用()定律来表示。
A.质量守恒
B.动量守恒
C.虎克
D.能量守恒