—  My cat’s really fat.— You ______ have given her so much food.

 -- Is it your brother playing football outside? -- It ________ be him. He is helping mother with dishes now.

— Do you know whose magazine is it?— I think it         be Lily’s, for we can find her name on the cover.

(2013·天津高考)No one ________ be more generous; he has a heart of gold.

Brain, like muscles, also needs some kind of exercise and nutrition. The more you use your brain in the right direction, the more you ________ enhance your ability.