This question tests your understanding of parallel structure with gerunds and infinitives. The sentence requires two verb forms that are grammatically parallel in construction.

The structure is: "The main idea behind [verb form 1] is [verb form 2]."

Step 1: Analyze the first blank. The word "behind" is a preposition. In English, prepositions (like behind, of, for, about) must be followed by a noun or a gerund (a verb form ending in -ing that functions as a noun). Therefore, the first blank must be filled with the gerund "demolishing".

Step 2: Analyze the second blank. The word "is" acts as a linking verb. The phrase after "is" should explain the "main idea". The correct structure for expressing purpose or intention after the phrase "the idea is" is the infinitive form "to + verb". Therefore, the second blank must be filled with the infinitive "to erect".

Final Answer: The main idea behind demolishing a lot of buildings is to erect new office blocks.

Related Concepts & Formulae

Gerunds: A gerund is the -ing form of a verb that functions as a noun. It is used after prepositions.
Formula: Preposition + $Verb+ing$ (Gerund)

Infinitives of Purpose: The infinitive form (to + verb) is used to express the purpose or intention of an action.
Formula: The idea/goal/plan + is + $to+Verb$