To estimate the expansion rate of the universe is a notoriously difficult problem because there is a lack of a single yardstick that all distances can be measured by.

To estimate the expansion rate of the universe is a notoriously difficult problem because there is a lack of a single yardstick that all distances can be measured by.

Estimating the expansion rate of the universe is a notoriously difficult problem because there is no single yardstick by which all distances can be measured.

Because there is a lack of a single yardstick to measure all distances by, estimating the expansion rate of the universe is a notoriously difficult problem.

A notoriously difficult problem is to estimate the expansion rate of the universe because a single yardstick is lacking by which all distances can be measured.

It is a notoriously difficult problem to estimate the expansion rate of the universe because by no single yardstick can all distances be measured.

##### 考题讲解

A选项：because之后跟着一个there be句型，其核心成分为“there is a lack”，即“这里有一个缺少”。但直接导致“估计速度是个难题的”应该是“没有标尺”这件事，而不仅仅是“这里有个缺少”。

B选项：正确。because之后的核心成分为“there is no yardstick”，即“这里无标尺”。

C选项：there is a lack错误同A。

D选项：此选项的字面意思为“因为标尺正在缺少，所以难题 是 预估速度这件事”。很明显，“正在缺少”这一进行时是不合理的，且“标尺缺少”导致的应该是“ 预估速度这件事 是 难题”，而不是“难题 是 预估速度这件事”。

E选项：主句中，it作形式主语，指代“预估速度”这件事。原因状语从句中，是一个倒装，还原一下为“all distances can be measured by no single yardstick.” 其字面意思为“所有的距离可以被 没有任何一个标尺 测量 ”。那么整句话的核心意思为“因为被测量，所以是难题”。但合理的应该是“因为没标尺，所以是难题”。