Prices reach equilibrium at the level at which quantity demanded quantity supplied.
A. is equal to
B. equal
C. equals to
D. is equal
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(答案→)A. is equal to
B. equal
C. equals to
D. is equal
解析：语法结构题。本句的难点是：be equal to意为“与…相等”，等于及物动词equal。任at which引导的定语从句中，主语为第三人称单数，而[B]项equal又没有词尾s，C.和D.两项的形式又不正确，所以[A]项正确，因其既合语法也合句意。本句的意思是：物价应在供求量相等的水平上达到平衡。