Explanation

Frequency of sound refers to the number of vibrations per second.

It is measured in Hertz (Hz) — 1 Hz = 1 vibration per second.

Explanation

"Oldest" is the superlative degree of the adjective "old".

Superlative form shows the highest degree among three or more things.

Explanation

Mandela led the struggle against racial segregation (apartheid) in South Africa.

He became the country's first Black president in 1994.

Explanation

Explorer and Firefox are web browsers used to access the internet.

Such programs are categorized as application software, designed for specific user tasks.

Explanation

The sum of two whole numbers is always a whole number — this is called closure property of addition.

Whole numbers include 0 and all positive integers (0, 1, 2, 3, ...).

Explanation

"Which" is used for things, not people.

It should be "who is very ill" because it refers to a person (uncle Jamal).

Explanation

"Sang" is the past tense of the verb "sing" and shows the action performed.

A verb expresses an action, event, or state of being in a sentence.

Explanation

Explanation

Subject Identification:

"They" is the subject of the sentence, as it performs the action (knew).

"Both" is an adjective modifying "they," emphasizing the duo.

The subject is who/what the sentence is about.

Here, "they" (the duo) knew each other.

Explanation

The correct sentence should be:

"The old headmaster was walking along the road when he fell down."

Preposition Error:

"Below" means underneath (e.g., "below the surface"), which doesn’t fit the context.

"Along" is correct here, meaning moving in a line next to the road.