In the following sentence, choose the correct verb form to replace the bracketed phrase: Tunnels (is built) to connect houses and places of worship, whereas innovative trick doors were attached to the underground cities.

Introduction / Context:

In English grammar, subject verb agreement ensures that plural subjects take plural verbs and singular subjects take singular verbs. Examination questions often test this rule by giving a sentence with the wrong verb form inside brackets and asking you to choose the correct alternative. In this sentence about tunnels that connect houses and places of worship, the focus is on recognising that the subject of the verb is plural and therefore requires a plural verb form. Understanding this rule is essential for clear, correct writing in academic as well as everyday contexts.

Given Data / Assumptions:

- The original sentence is: Tunnels is built to connect houses and places of worship whereas innovative trick doors were attached to the underground cities.

- The phrase inside brackets is is built, and we must replace it with the grammatically correct form.

- The subject of the verb in the first clause is tunnels, which is clearly plural.

- We assume standard modern English usage, where plural countable nouns take plural verbs in the present and appropriate plural forms in the past.

Concept / Approach:

The core concept is agreement between subject and verb in both number and tense. A plural subject such as tunnels cannot be followed by a singular verb like is. In addition, the second clause of the sentence uses the past tense were attached, which suggests that the first clause should also be in the past passive form in order to keep the time reference consistent. The correct past passive form for a plural subject is were built. This form matches both the plural subject and the past time reference already used in the sentence.

Step-by-Step Solution:

Verification / Alternative check:

A quick way to verify the answer is to test each option in the sentence and see which one sounds both grammatical and parallel. Was built uses a singular auxiliary and clashes with the plural subject tunnels. Were building suggests an ongoing action and changes the meaning of the sentence. Leaving the phrase as is built keeps the subject verb mismatch. Only were built matches the plural subject and aligns with the past passive form already present in were attached, so this option passes both agreement and parallelism checks.

Why Other Options Are Wrong:

Option was built is wrong because was is singular and does not agree with the plural subject tunnels. Option were building is wrong because it introduces a continuous aspect that is not required by the context and does not match the completed action suggested by were attached. The option no improvement is wrong because the original phrase is built already contains an error in agreement and tense consistency. Therefore these alternatives are grammatically inferior to the correct choice.

Common Pitfalls:

A common mistake in such questions is to focus only on the bracketed phrase and forget to examine the subject and the rest of the sentence. Many learners also confuse was and were, choosing was out of habit even when the subject is clearly plural. Another frequent error is ignoring tense consistency between clauses, which leads to mixed time references that sound awkward or incorrect. Careful reading of the entire sentence and conscious checking of agreement help avoid these problems.

Final Answer:

The correct verb phrase that should replace the bracketed words is were built, because it agrees with the plural subject tunnels and matches the past passive form used later in the sentence.