"Larry,I can't give you an answer ---- I think it over," sai

应该选before
首先排除after,如用after后面应改为“I thought it over”,表示“想过以后”应用过去时态；when表示“当……时候”,用在此处句意不通顺；最易错选的是unti,“not…until”表示“直到（某时）（某动作）才（发生）”,not…until 用在否定句中,谓语动词通常是瞬间性动词,如open, start, leave, arrive, finish, stop等,强调主句动作开始的时间,think不是瞬间动词.故选before,句意为：拉里,在我仔细考虑之前,我不能给你答复.
希望对你有帮助!