because a is a number more than or equal to 1 , so a could be equal to 2, so k could be equal to 6 . i can't rule out the possibility of 6 as the value of k.
how to explain WHY k could not be 6? ( in another word, why a could not be 2?) and if k could be 4 or 6, i think the answer should be E, for two solution can not be determinded.