You ought to have put the milk in the refrigerator, and now ________sour
A.it smellB.it is smeltC.it smellsD.it is smelling在线课程C
考查时态。句意：你应该已经把牛奶放进冰箱了，可是现在闻起来酸了吧！结合语境可知下文描述的是现在客观存在的状态，故用一般现在时态，选C。