题目

 After he failed the exam, he tried his best to ____ and then made a proper plan. A.gather his thoughts  B. collect his ideas   C. use his thoughts   D. collect his thoughts 答案：D下列叙述正确的是  A．道尔顿发现了元素周期律，制订了第一张科学的元素周期表  B．短周期中包含了18种原子  C．元素的性质呈现周期性变化的根本原因是核外电子排布的周期性变化  D. 周期表中所有元素，都是从自然界中发现的