Explanation

The clear sentence is: "Too many tourists visit the cave each year. They could possibly damage the rock formations."

It correctly uses "they" to refer to tourists and clearly states the potential issue.

Explanation

"I'm at least as qualified as, if not more than, the other candidate."

"At least as qualified as" is a standard comparative construction.

"If not more than" adds the nuance that you might actually be more qualified.

The sentence flows logically and grammatically.

Explanation

The correct phrase is: "Although the sky was grey, we did go on our picnic."

"Did go" is the proper past tense form used for emphasis in this context.

Explanation

He is "vexed with" me for opposing him.

This phrase means he is annoyed or irritated with me.

Explanation

The correct tag question is: "Shouldn't they?"

It confirms the positive statement "They should have worked hard" by using the negative form of the auxiliary verb in the tag.

Explanation

"Listen and practice" is a phrase.

It lacks a subject and a complete predicate, so it does not form a complete sentence.

Explanation

The correct use of tenses is: "Your mother had been waiting for you when you went to your friend's house."

This sentence correctly uses the past perfect continuous tense to describe an action that was ongoing in the past before another past action.

Explanation

"Blow" is a transitive verb.

It requires a direct object to complete its meaning, as in "She blew the horn."

Explanation

The phrase "that without which one cannot do" is a definition of the word "indispensable", which means essential, necessary, or crucial.

Something that is indispensable is absolutely necessary or required, and cannot be done without it.

Explanation

• The phrase "took against" is an idiomatic expression that means to take a dislike or have a negative reaction to someone.

So, the sentence would read:

• "He took against me the minute we met. But I'm not worried; I can take care of myself."