It is know to all that it takes years of ( )practice to gain the skill of( )expert.

B.
years of practise(practise不可数）类似的years of hard work
expert原因开头加a.

B/;an

It is known to all that it takes years of practice to gain the skill of an expert.

众所周知，成为一名专家所应具备的技巧能力是需要多年的实践才能获得的。

B/;an，因为practice 不可数，前面不能用 a， an expert 是泛指概念。

It is known to all that it takes years of practice to gain the skill of an expert.

众所周知，要拥有一名专家级的技能是需要经过多年的实践的。