He has made progress in his English, _______ makes his father very happy.

A．who

B．that

C．what

D．which

系统接线如图4.5-26所示，各元件参数为G1、G2：30MVA，z，T1、T2：30MVA，Uk%=10；选SB=30MVA时，线路标幺值为XL=0.3，X0=3XL。当系统在f点发生A相接地短路时，短路点短路电流为（）kA。

A.2.21

B.1.199

C.8，16

D.9.48