Explanation

Leukocytes, or white blood cells, are key components of the immune system that defend the body against infections.

They identify, attack, and destroy harmful pathogens like bacteria and viruses.

Explanation

Checking prior knowledge helps the teacher connect the new lesson with what students already know.

It ensures that the foundation is strong before introducing new concepts.

Explanation

Roman numeral LVI =

L = 50, V = 5, I = 1

So, 50 + 5 + 1 = 56.

Explanation

Polyisoprene is the synthetic version of natural rubber.

It has similar properties.

It is commonly used to make rubber products.

Explanation

Given that 3 women = 2 men in terms of work:

Work done by 15 men in 1 hour = 1 / (21 * 8)

Work done by 1 man in 1 hour = 1 / (15 * 21 * 8)

Work done by 1 woman in 1 hour = (2/3) * (1 / (15 * 21 * 8))

Work done by 21 women in 1 hour = 21 * (2/3) * (1 / (15 * 21 * 8))

= 14 / (15 * 21 * 8)

= 1 / (15 * 9 * 8) (simplified for 21 women working)

If 21 women work for 6 hours a day:

Work done by 21 women in 1 day = 6 * (1 / (15 * 9 * 8)) * (15 * 9)

= 6 / 8

To complete the work:

Number of days = 8 / 6 * 21

= 28

None of the given options directly match the calculation approach provided, but recalculating properly considering the work equivalence and hours:

15 men * 21 days * 8 hours = 2520 man-hours

Given 3 women = 2 men, 21 women = 14 men equivalent

Let x be the number of days for 21 women working 6 hours/day:

14 men * x days * 6 hours = 2520 man-hours

84x = 2520

x = 30

Explanation

The phrase "settle down" means to start living a stable and orderly life, often by getting married.

It's a common idiom used when someone is expected to stop moving around or partying and adopt a more settled lifestyle.

Explanation

To find the LCM (Least Common Multiple) of 10, 16, and 25:

Prime factorization:

10 = 2 × 5

16 = 2⁴

25 = 5²

Now take the highest power of each prime:

2⁴ (from 16)

5² (from 25)

LCM = 2⁴ × 5² = 16 × 25 = 400

Explanation

George Cantor, a German mathematician, is known as the father of set theory.

He introduced the concept of infinite sets and their cardinalities in the late 19th century.

Explanation

Petroleum has the highest energy density among fossil fuels, making it the richest source of energy.

It is widely used in transportation, industry, and power generation due to its high calorific value.

Explanation

Present perfect passive structure: has/have been + past participle

Active: I have written a story.

Passive: A story has been written by me.