Laughter, along with an active sense of humor, ______ protect you against a heart attack, according to a recent study.

A．have helped

B．helped

C．help

D．helps

不属于雄性河蟹的外观特征的是（）

A.腹部狭长

B.腹部浑圆

C.生殖孔开口于腹甲第7节

D.腹部附肢2对