问题
选择题
| The teacher raised his voice, but still couldn't make himself _____. |
A. heard B. hearing C. to be heard D. hear |
答案
答案：A
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| The teacher raised his voice, but still couldn't make himself _____. |
A. heard B. hearing C. to be heard D. hear |
答案：A
由关系R和S通过运算得到关系T，则所使用的运算为