Itisthought_ofseasonalvariationisthechangeinthewindpatternsoftheNorthernHemisphere.
(A)whattheprincipalcause
(B)oftheprincipalcause
(C)thattheprincipalcause
(D)theprincipalcauseis
答案及解析,请问这是什么从句?谢谢您帮忙