Inversion - Sentence starting with rarely

This is from my English Cambridge Proficiency Book. In Unit 8 about inversions, I had to rewrite the sentence:

Original sentence:

It is very difficult for town centre redevelopment to achieve a harmonious balance between old and new

My answer:

Rarely is it easy for town centre redevelopment to achieve a harmonious balance between old and new

Correct question (according to the book):

Rarely do/will/can town centre redevelopments achieve a harmonious balance between old and new.

I wonder why my answer isn't acceptable.

Is there anything fundamentally wrong with my sentence?

Thanks