三题名词的语法选择题!并说明理由

三题名词的语法选择题!并说明理由
1.Most of the houses in the village were burnt to ______ during the war.
a.an ash b.the ash c.ash d.ashes
2.The students at colleges or universities are making ______ for the coming New Year.
a.many preparations b.much preparation c.preparations d.preparation
3.Painting in _____ is one of their spare-time activities.
a.oil b.an oil c.oils d.the oil
答案分别是DCC，很多人最后一题都选择A，但我还是没办法搞清楚到底谁对谁错？