how much it costs
how much it cost
how much did it cost
how much does it cost
本题考查宾语从句的语序。forgot后接宾语从句,宾语从句需使用陈述语序。主句时态为一般过去时,从句也应使用一般过去时,B选项符合题意。句意:——这套家具多少钱? ——我忘了多少钱。