影响混凝土强度的因素有（）。
A.砂率
B.水灰比
C.骨料性能
D.（A+B+C.
参考答案：B
Tom didn’t go hiking with his classmates ________his illness.
A．because
B．because of
C．as
D．since
比较直接选举与间接选举的区别。
