I think this sofa is ___ than that one.

选择一项：

a.
expensiver


b.
more costly


c.
cost more


d.
costlier

expensive “昂贵的”，该形容词比较级为 more expensive；costly “昂贵的”，该形容词比较级为 more costly。因此，此题选B， more costly 最合适，句意为：我认为这个沙发比那个贵。