Sentence Correction

Hello.

The below sentence is choice B of question 80 in verbal review 2.

"X cannot detect when Y is present"

I eliminated the choice because I think that "detect" need an object. In the choice, "when y is present" is adverb modifying "detect", so we do not know what X cannot detect.

Is my interpretation correct?

Thank you.

In the sentence "X cannot detect when Y is present", what X cannot detect is "the presence of Y". The solution below is taken from the GMATFix App.



-Patrick