In the following question, a sentence is divided into few parts. Rearrange these parts and identify the correct sequence making the sentence grammatically and contextually correct.

(A) levels in the city

(B) on reducing pollution

(C) the new law had

(D) a significant effect

To find the correct sequence, let us analyze the grammatical and logical connections between the sentence fragments:

1. Identify the starting fragment: Fragment (C) "the new law had" contains the subject ("the new law") and the transitive verb ("had"). This serves as a natural starting point for the sentence.
2. Find the object of the verb: The verb "had" requires a direct object. Fragment (D) "a significant effect" completes this requirement, forming the phrase "the new law had a significant effect".
3. Connect the prepositional modifier: The noun "effect" is commonly followed by the preposition "on" to indicate what is being influenced. Fragment (B) "on reducing pollution" fits perfectly here, giving us "had a significant effect on reducing pollution".
4. Complete the sentence: Fragment (A) "levels in the city" naturally follows "pollution" to form the compound noun phrase "pollution levels in the city" and specify the location.

Combining these steps in order, we get: (C) the new law had + (D) a significant effect + (B) on reducing pollution + (A) levels in the city.

The complete, grammatically correct sentence is: "The new law had a significant effect on reducing pollution levels in the city."

Therefore, the correct sequence is CDBA.