The swimming pool will not be open until nine o'clock because they________the water．So we have to wait outside the swimming pool．
A.have changed
B.change
C.are changing
D.had changed
网友回答
C解析分析：从will not be open可以知道是以现在为立足点来考虑，因为他们现在“在换水”，作为“游泳池要到九点钟才对外开放”的理由是合理的。