【判断题】吃东西 使 唾液分泌量会增多是无条件发射。
【简答题】姜黄素(分子式为C 21 H 20 O 6 )具有搞突变和预防肿瘤的作用,其合成路线如下: 已知 ① ② 请回答下列问题: (1)D中含有的官能团名称 。 (2)试剂X为 。 (3)姜黄素的结构为 。 (4)反应D—E的化学方程式为 ,其反应类型是 。 (5)下列有关G(C 8 H 8 O 3 )的叙述不正确的是 (填正确答案编号)。 a.能与NaHCO 3 溶液反应 b.能与浓溴水反应 c.能与...
【判断题】90°车刀(偏刀),主要用来车削工件的外圆、端面和台阶。
【简答题】听下面一段材料,回答第1-3题。 1. How does the woman keep in touch with her family? A. By letter. B. By computer. C. By telephone. 2. What does the man advise the woman to do? A. Talk with people online. B. Learn h...
【简答题】听下面一段材料,回答第1至3三个小题。 1. What is the two speakers'common hobby? [ ] A. Performing on a concert. B. Playing a musical instrument. C. Studying the theory of music. 2. How often does the man take part ...
【单选题】在我国被称为"动物王国" 与 “植物王国”,是我国最重要的动物和植物基因库。
【单选题】The essence of systematic sampling is that:
A.
Each element of the population has an equal chance of being chosen
B.
Members of various strata are selected by the interviewers up to predetermined limits
C.
Every nth member of the population is selected
D.
Every element of one definable sub-section of the population is selected.
【多选题】Which of the following are disadvantages of systematic sampling? ( )
A.
The sample chosen might be biased.
B.
Some samples have a zero chance of being selected so the sampling method is not completely random.
C.
Prior knowledge of each item in the population is required.