Thank you for all your hard work .I don't think we ___ it wi

Thank you for all your hard work .I don't think we ___ it without you.
Thank you for all your hard work.I don't think we ___ it without you.
A can manage
B could have managed
C could manage
D can have managed
句中没有任何时间状语,为什么答案是B?为什么用虚拟啊?
think之后要用虚拟吗?