Flies that did not avoid the odor when tested individually were less likely than the other flies to avoid the odor when tested in a group.单独测试时没躲开气味的苍蝇在群组测试的时候更不容易躲开。-取反就是 单独测试没躲开的苍蝇在群组测试时候更会躲开，就加强结论，这就证明原来的是削弱结论的。所以不是C
However, when diatoms are observed under constant conditions in a laboratory, they still display periodic behavior, continuing to burrow on schedule for several weeks.
This is not to suggest that the period of this biological clock is immutably fixed.
根据前面不太好判断，因为前面说是几周是不变的。但是后来又说到不是完全fix的。。所以还是选short比较严谨
membership：
[C + sing./pl. v.] （统称）会员，成员；会员人数；成员数
the members, or the number of members, of a group, a club, an organization, etc.
The membership has/have not yet voted.
会员还没有进行投票。
The club has a membership of more than 500.
俱乐部的会员人数超过了500名。
Since 1980, productivity improvements in manufacturing have moved the United States from a position of acute decline in manufacturing to one of world prominence.
扫描二维码，直接登录
but在此处是连词（如果是介词，其身后只能连接名词或者名词短语），其身后连接的是条件状语（only if）。但是，在but身前没有能和这个条件状语平行的其它状语，所以only if前的but是错误的，应该去掉。
大概的意思是：OPEC组织一直被认为要通过减产的方式来提升油价，但是它们现在声明：仅当（only if）非OPEC国家减产时，它们才会减产。