'Fatter' 是 'fat' 的比较级。

'Fatter' is the ___comparative of 'fat'.

'More difficult' 是 'difficult' 的比较级。

'More difficult' is the ___comparative of 'difficult'.

'Happier' 是 'happy' 的比较级。

'Happier' is the ___comparative of 'happy'.