They began to believe that their way of doing business was failing, and their incomes would therefore shortly begin to fall as well.
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【正确答案】 1、正确答案：宾语从句引导词缺失，应在and后面加上that。    
【答案解析】解析：此处是由that引导的两个并列的宾语从句，作believe的宾语，that不能省略。