Scientists are convinced ________ the positive effect of laughter ________ physical and mental health.

A．of; at

B．on; at

C．by; in

D．of; on

以下关于家族性慢性良性天疱疮临床表现的描述正确的有()

A．出生即发病

B．典型皮损为尼氏征阳性的松弛性水疱

C．可累及口腔粘膜

D．皮损愈后一般不留瘢痕