元素周期表是学习和研究化学的重要工具．回答下列问题：

（1）元素周期表中不同元素间最本质的区别是____ （填字母）．

A．质子数不同   B．中子数不同   C．相对原子质量不同

（2）1869年，____ （填字母）发现了元素周期律并编制出元素周期表．

A．张青莲   B．门捷列夫   C．拉瓦锡

（3）元素周期表中氟元素的有关信息如图所示，下列说法正确的是____ （填字母）．

A．氟属于金属元素   B．氟的原子序数是9   C．氟的相对原子质量是19.00g

（4）氟原子结构示意图为，氟原子在化学反应中易____ （填“得到”或“失去”）电子，由钠元素和氟元素组成的化合物氟化钠的化学式为____ 　．

函数$f\left(x\right)=lgx+\sqrt{1-2x}$的定义域为（   ）

$\frac{1}{2}$ 的相反数是（　　）

    Long, long ago, there was a man. He lived in a village. One day, he went to a market and did some shopping. He suddenly found two boxes. Both the two boxes were very beautiful and the man loved them. So he bought the two boxes and took them home.

    He looked at the boxes. He didn't know how to use the boxes. He thought it over and decided to do something special. He decided to put all of his happiness in the yellow box and all of his sadness in the black box. He thought that it was funny to do that.

    A few days later, the yellow box was very heavy, but the black box was still light.

    He didn't know why. He decided to open the two boxes and found out the truth. He opened the yellow box and found nothing. Then he opened the black box and saw a hole (洞)at the bottom(底).