_______ your valuable help, we couldn’t have finished the experiment ahead of time.
If it were not for
Had it not been for
Were it not for
If it has not been for
本题考查虚拟语气和倒装
B选项，由主句谓语可知本题是与过去事实相反的虚拟条件句，条件从句谓语用“had+过去分词”，如果省略if，从句要用倒装语序，即把had移到主语前面。综上，B选项正确。
A选项，与题意不符，故排除。
C选项，与题意不符，故排除。
D选项，与题意不符，故排除。
故正确答案为B 项。
