减数分裂过程中染色体数目减半的原因是（　　）

被吸在竖直的磁性黑板上的小棋子，虽然受到竖直向下的重力作用，但不会掉下来，其主要原因是（     ）

用所给动词的正确时态、语态填空
⑴Be quiet, please ! The baby____ (sleep).
⑵—What does Frank often do at weekends?
—He often____ (go) to the cinema with his parents.
⑶Andy ____ (argue) with his brother when I went to see him yesterday.
⑷She ____ (be) to her hometown several times.
⑸____ (play) computer games too much is bad for your eyes.
⑹Lucy tried ____(not make) too many mistakes in the exam.
⑺He ____  to us as soon as he gets there. (write)
⑻By the time I got outside, the bus____ (leave).
⑼I'm sure I ____  her four years ago. (see)
⑽In summer,  Miss Xia would rather ____  (stay) at home than go out.

设函数y=f(x)在(-∞,+∞)内有定义，对于给定的正数k，定义函数：$f_k\left(x\right)=\left\{\begin{array}{c}f\left(x\right),\left(f\right(x)\le k)\\ k,\left(f\right(x)>k)\end{array}\right.$,取函数f(x)=2-x-e^-x，若对任意的x∈(-∞,+ ∞)，恒有f_k(x)＝f(x)，则(    )

氯化钠是我们日常生活的必需品，也是重要的化工原料，可以制备一系列物质。氯化钠的用途如图所示，下列说法错误的是（ ）