Many scientists are sure ________will be useless learning a theory without practice．A.thatB.that itC.it thatD.there
网友回答
B解析在are sure的后面是一个宾语从句，从句子的结构来看，宾语从句缺少主语，应为句子选一个主语，如选that则that指代不明，或that用做连词的话，则句子仍无主语；如选C则结构不对，故考虑选用that it；如选D则构成there be句型，则句末的动名词短语不能在句中做句子成分。故本题的考查点是it作形式主语。宾语从句相当于Learning a theory without practice will be useless．