在核酸中含量相对恒定的元素是（）
A.氧
B.氮
C.镁
D.碳
E.磷
参考答案：E
看图写出正确的时间。
1. -What's the time? -It's 5:00/________.
2. -What's the time? -It's 4:30/________.
3. -What's the time? -It's 11:00/________.
4. -What's the time? -It's 1:30/________.
5. -What's the time? -It's 3:30/________.
6. -What's the time? -It's 8:00/________.
7. -What's the time? -It's 3:00/________.
三棱锥的中截面面积与该三棱锥底面面积的比为（ ）
A．1：2
B．1：3
C．1：4
D．1：5
-It's 5:00/________.
-It's 4:30/________.
-It's 11:00/________.
-It's 1:30/________.
-It's 3:30/________.
-It's 8:00/________.
-It's 3:00/________.