问题
选择题
It is time for class. Will you please get the blackboard__________?
A.to clean
B.cleaning
C.cleaned
D.having cleaned
答案
答案:C
考察使役动词用法。Have get等使役动词后面加动词的过去分词,表示把某事做了。根据题意,请你把黑板擦了好吗?选C。
It is time for class. Will you please get the blackboard__________?
A.to clean
B.cleaning
C.cleaned
D.having cleaned
答案:C
考察使役动词用法。Have get等使役动词后面加动词的过去分词,表示把某事做了。根据题意,请你把黑板擦了好吗?选C。
设有关系R,S和T如下:关系T是由关系R和S经过哪种操作得到的
R
A | B | C |
1 | 2 | 3 |
4 | 1 | 6 |
3 | 2 | 2 |
B | D | E |
4 | 1 | 6 |
2 | 7 | 1 |
A | B | C | D | E |
1 | 2 | 3 | 7 | 1 |
3 | 2 | 4 | 7 | 1 |
A.R∪S
B.R-S
C.R×5
D.R
S