______是指在一定的时间和相对稳定的条件下，生态系统内各部分(_爱下问搜题

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问题填空题

______是指在一定的时间和相对稳定的条件下，生态系统内各部分(生物、环境和人)的结构和功能均处于相互适应与协调的动态平衡。

答案

参考答案：生态平衡

补全对话，情景问答

根据I栏的问句，从Ⅱ栏中找出与之相应的答语。

( )1. Were the twins at the party last night?
(     )2. Was Mr Li at work or away yesterday?
(     )3. How many people were there at the meeting?
(     )4. Why was Liz not here last night?
(     )5. Were you late this morning?
(     )6. What did you have for breakfast?

A. She was ill.
B. No, I was quite early.
C. Yes, they were.
D. I had bread and milk.
E. He was at work.
F. About twelve.

填空题

如图所示，某个物体在F₁、F₂、F₃和F₄四个共点力作用下处于静止状态，若F₄的方向沿逆时针转过90°而保持其大小不变，其余三个力的大小和方向均保持不变，则此时物体所受到的合力大小为______．