I had been puzzled over the problem for over an hour without any result, ____ all at once the solution flashed across my mind.

A．while

B．when 　　

C．then 　　

D．as

以下哪个疾病临床上仅有继发皮损及瘙痒症状而无原发性皮肤损害的皮肤病（）

A.湿疹

B.瘙痒症

C.痒疹

D.特应性皮炎

E.结节性痒疹