the boy is to learn to talk.
A. How slow
B. How slowly
C. What a slow
D. What a slow way
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(答案→)A. How slow
B. How slowly
C. What a slow
D. What a slow way
解析：这是感叹句，还原为：The boy is slow to learn to talk和The boy is to learn to talk slowly语法上都没有什么错误，但根据语境和常识判断，前一句是正确的。