问题补充：
for the boy to talk with his mouth full!
A. What bad manners they are
B. How bad manners they are
C. What bad manners it is
D. How bad manners it is
答案：
A. What bad manners they are
B. How bad manners they are
(答案→)C. What bad manners it is
D. How bad manners it is
解析：句意：一个男孩满嘴是食物说话多么不礼貌呀！这里是感叹句被感叹的是名词manners，所以用what，what+形容词+名词+主语+谓语，它的陈述句是it is bad manners for the boy to talk with his mouth full，这里it是形式主语，for sb to do 是真正的主语，故选C。
