ku is substituted for kim before the vibhakti at ,
As क्व गमिष्यसि, क्व भोक्ष्यते ॥ The substitute कु of the last rule, before the affix अ would have become क्व, but that it would have caused guna, hence this separate substitute. Had the sutra been किमोऽड्वत् it would not have included the अकच् augmented किम् ॥,