Mary is going to live with his parents. they havedecided to buy a bigger house because their house is ______. a.not big

Mary is going to live with his parents. they havedecided to buy a bigger house because their house is ______. a.not big enough for them b. too small for them to live ,我知道选a. 请问 b 哪错了.是少了一个in 吗?