生态平衡是指生态系统中各种生物( )
A:数量不改变
B:比例不改变
C:数量和所占比例总是相对平衡
D:数量暂时平衡
电影院呈阶梯或下坡形状的主要原因是 ( )
请用正楷字将下面的语句准确、规范、美观地抄写在田字格里。
为心中的理想而奋斗。
The chief engineer of the high-speed railway ________ Shanghai to Beijing always devotes most of his time ________ his work.
— Laura, we've decided on a trip this afternoon. Will you join us?
— I’m afraid not. I have a composition .