his being not able to come并不错

有一读者问一道选择题：

He apologized for _____.

A. his being not able to come.　B. his being able not to come.

我觉得A和B都不对，应填his not being able to come，不知对否？

A与你的填法都对。如果愿意还可以填his being unable to come。