与钙剂合用防治佝偻病的是

A．维生素A
B．维生素B₁₂
C．维生素B₆
D．维生素D
E．维生素K

IP multicasting is a set of technologies that enable efficient delivery of data to many locations on a network. Rather than making multiple copies of a message intended to be distributed to multiple recipients at the (71) of origin of a message, multicasting initially sends just one (72) and does not copy it to the individual recipients until it reaches the closest common point on the network, thereby (73) the bandwidth consumed. Network performance is significantly (74) because it isn’t bogged down with the processing and transmission of several large data fries; each receiving computer doesn’t have to (75) the transmitting server for the file.

A．dropped

B．proved

C．improved

D．upgraded