Which of the following is a well-defined set?

1. The set of tall students in a class

2. The set of good books

3. The set of prime numbers less than 20

4. The set of beautiful flowers

Which of the following is an improper fraction?

1. $\dfrac{3}{4}$

2. $\dfrac{5}{8}$

3. $\dfrac{9}{7}$

4. $\dfrac{1}{3}$

The ratio of 3 km to 750 m is

1. 1:250

2. 4:1

3. 1:4

4. 250:1

Which of the following is a variable?

1. 5

2. x

3. 3 + 4

4. 8