用金属型铸造时，金属型为什么要预热？
参考答案：
预热作用：
（1）避免浇不足，冷隔缺陷
（2）避免金属型强烈热冲击，影响金属型寿命
-- I have warned him of the danger but he insisted on going there alone.
-- What a _______ boy!
A．stubborn
B．devoted
C．curious
D．helpful
用所给单词的适当形式填空。
1. He is (write) an email to his father now.
2. That big ship (sink) into the sea many years ago.
3. He was very excited when he (read) the letter from his mother.
4.I think playing basketball is (excited).
5. Did you see Mei Ling (play) computer games when you came in?
g ship (sink) into the sea many years ago. 