When it comes to “empty-nest syndrome(综合症)”, parents are advised to _______regular contact with children through phone calls, emails, texts or video chats.

A．substitute

B．sacrifice

C．associate

D．maintain

转炉生产的钢水氢含量（）电炉和平炉的钢水。

A.小于

B.等于

C.大于

D.大于等于