My friend ______ from the UK. She studied at Oxford for four years, during which we communicated with each other on the Internet.

A．stepped

B．has returned

C．went

D．has learned

安全约束发电计划编制要求支持的机组组合包括（）。

A.完全机组组合

B.部分机组组合

C.无机组组合

D.上述三项都是