【正确答案】 D

【答案解析】[解析] that引导的从句作主语。一般情况下将主语从句放在句末，用无人称代词it作形式主语，如本句可改写为It is generally agreed upon by authorities in the field that the dog was the first animal to be domesticated。能引导主语从句的除that外，还有选择连词whether...or not，疑问副词why/when/where和疑问代词who/what/which等。