A:if
B:whe
C:that
D:which
—Haha, it's free for all the people today. So you _________ buy tickets.
经过人工改造的生态系统往往是不稳定,这类生态系统的平衡需要____ 来维持。
我会填。
①是:共____画 第八笔是____。
②空:共____笔,第二画是____。