答案B
分析:The more careful you are,the fewer mistakes you'll make.这是“the+比较级,the+比较级”的句型,前面表示“条件”,谓语动词用一般现在时;后面表示“结果”,谓语动词用一般将来时。因为第一句的动词是are,所以要用形容词careful.(延伸)本句可以表达为:If you are more careful in the exam,you'll make fewer mistakes.=Be more careful in the exam,and you'll make fewer istakes.也就是说,如果在含有if(如果)的复合句中,主句和从句里都有比较级,或在“祈使句+and/or”的句型中两句里都有比较级的话,就可以用“the+比较级,the+比较级”的句型来表达相同的意思。