Correct

难于

/ nán yú / 
Strokes

Definition

difficult

Example

As an Adverb

hard/difficult to do sth

1
Nányú
难于
zòuxiào
奏效

be hard to get result

2
Nányú
难于
qǐchǐ
启齿

too shy to speak out; too embarrassed to mention

As an Adverb

harder than; more difficult than

3
Shǔdào
蜀道
zhī
之
nán
难
  
,
nányúshàngqīngtiān
难于上青天
  
。

The road to Sichuan is as difficult as climbing to heaven.

4
Dìyī
第一
tí
题
nányú
难于
dìèr
第二
tí
题
  
。

The first question is more difficult than the second.