He did well in the examination. He _____ hard at his lessons..  

A．must have worked

B．can have worked

C．will have worked

D．has worked

关于经线和经度的叙述，正确的是（）

A.同一经度，纬度越高，越在北方

B.春秋分日，同一经线上同时日出日落

C.东经20°与西经20°经线区时相差2h，地方时相差2h40min

D.因每条经线的长度相等，故不同纬线上经度相差1°的弧长均是相等的