Email,as well as telephones ________ an important part in daily communication.[ ]

A. is playing  

B. have played

C. are playing  

D. play

工业碳酸钠的测定，以盐酸标准溶液作为滴定剂，以溴甲酚绿-甲基红混合指示剂作为指示剂，在室温下滴定至试验溶液由绿色变为紫红色，即为终点。( )