All 5 children of the Blacks, _______ are now working abroad, will come back for their parents’ 50^th wedding anniversary.

A．two of whom

B．two of them

C．both of whom

D．both of them

阀控密封铅酸蓄电池安全阀具有（）功能

A.调节电池内外压力

B.过滤酸雾

C.阻燃

D.防电池内部污染