单选题 We decided to have an addition built onto our house when it became too ______ to have six family members living in just two bedrooms.

【正确答案】 D

【答案解析】[解析] 单词形态：形容词
1．空格内需填入一个可以被副词too修饰的形容词，因此名词B选项和动词C选项排除。
2．形式主语it引导的内容不是沮丧的对象，而是“令人沮丧的”，因此现在分词D．frustrating为正确选项。