环焊缝透照时，有效透照长度相关于（）
A、外径与壁厚
B、焦距
C、壁厚与外径之比
D、b和c
参考答案：D
用所给单词的适当形式填空。
1. My aunt's son speaks _____ (China) very well.
2. It's Monday. My uncle is _____ (real) busy today.
3. These kids usually have three _____ (class) in the morning.
4. It is Tuesday, December _____ (ten).
5. His grandfather is a great _____ (music).
Many of the students turned a deaf ear to his advice, they knew it to be valuable．
A．as if
B．now that
C．even though
D．so that
