下面这道题目请考生自己按照上面所提到的思路去分析，不难得出正确答案为D。
4. Lacking information about energy use, people tend to overestimate the amount of energy used byequipment, such as lights, that are visible and must be turned on and off and underestimate thatused by unobtrusive equipment, such as water heaters.
(A)equipment, such as lights, that are visible and must be turned on and off and underestimate that
(B)equipment, such as lights, that are visible and must be turned on and off and underestimate it when
(C)equipment, such as lights, that is visible and must be turned on and off and underestimate it when
(D)visible equipment, such as lights, that must be turned on and off and underestimate that
(E) visible equipment, such as lights, that must be turned on and off and underestimate it when
虽然语法改错部分看起来容易把握，但在实际解题过程中，考生经常顾此失彼，在几个选项间徘徊犹豫半天依然难以确定最终答案。GMAT语法改错的难度一方面来自题干，另一方面来自选项，因为我们要依靠选项之间的差别来获得信息、寻求提示，其实这就反映了GMAT语法改错答案的特点，也就是需要我们善于运用排除法，挑选出相对准确而有效的那一个。
攻克GMAT语法改错：死原则，活运用（3）
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