One day Mrs Jones goes shopping alone(独自地). When Mr Jones comes home in the evening, she begins to tell him about a beautiful dress.  “I see it in a shop this morning,” she says, “and…” “And you want to buy it,” says Mr Jones, “how much is it?” “Fifteen pounds.” Every evening, when Mr Jones comes back from work, his wife goes on talking only about the dress. And at last, after a week, he says, “Oh, buy the dress! Here's the money!” She is very happy. But the next evening, when Mr Jones comes home and asks, “Have you got the famous(著名的) dress?” She says, “No.” Why not?” he says. “Well, it is still in the window of the shop after a week. I think nobody else wants this dress. So I think it is not a good one!”

两物体分别从不同高度自由下落，同时落地，第一个物体下落时间为t，第二个物体下落时间为t/2，当第二个物体开始下落时，两物体相距（   ）

几种短周期元素的原子半径及主要化合价如下表：

下列说法正确的是（   ）