什么叫煤层“三要素”？
参考答案：
煤层走向、倾向、倾角为煤层三要素。
— Hi, Jane. What time do you get up _____ weekdays?
— I usually get up _____ about six o'clock. [ ]
A.in; at
B.on; at
C.at; at
D.on; on
植物体内四种色素的含量和比例随植物种类、生境条件的不同而有所差异。( )
