I would have gone with you，but I（ ）too tired．

I would have gone with you，but I（ ）too tired．

was

had been

must be

must have been

本题考查虚拟语气和时态。

本题分为两部分，前半句“would have done”是对过去的虚拟。后半部分表示真实原因，也应用过去式was。

句意：我本来应该跟你走的，但是我太累了。本题混淆项为B，但B不是表示事实，所以选A。