Fill in the blanks: a. Four cups = 1000 ml. b. Four cups = _ litre. c. One cup is of a litre. d. Two cups are of a litre. e. Three cups are _ of a litre.

$$
4 \text{ cups} = 4 \times 250 \text{ ml} = 1000 \text{ ml}
$$

$$
1000 \text{ ml} = 1 \text{ litre}
$$
So, four cups = $\boxed{\text{1}}$ litre.

$$
1 \text{ cup} = 250 \text{ ml}
$$

$$
\frac{250 \text{ ml}}{1000 \text{ ml}} = \frac{1}{4}
$$
So, one cup is $\boxed{\frac{1}{4}}$ of a litre.

$$
2 \text{ cups} = 2 \times 250 \text{ ml} = 500 \text{ ml}
$$

$$
\frac{500 \text{ ml}}{1000 \text{ ml}} = \frac{1}{2}
$$
So, two cups are $\boxed{\frac{1}{2}}$ of a litre.

$$
3 \text{ cups} = 3 \times 250 \text{ ml} = 750 \text{ ml}
$$

$$
\frac{750 \text{ ml}}{1000 \text{ ml}} = \frac{3}{4}
$$
So, three cups are $\boxed{\frac{3}{4}}$ of a litre.

$$
4 \text{ cups} = 4 \times 250 \text{ ml} = 1000 \text{ ml}
$$

$$
\frac{1000 \text{ ml}}{1000 \text{ ml}} = 1
$$
So, four cups are $\boxed{\text{1}}$ of a litre.

$$
3 \text{ cups} = 3 \times 250 \text{ ml} = 750 \text{ ml}
$$
$$
\frac{1}{2} \text{ of a litre} = \frac{1}{2} \times 1000 \text{ ml} = 500 \text{ ml}
$$

$$
750 \text{ ml} > 500 \text{ ml}
$$
So, 3 cups $\boxed{>}$ $\frac{1}{2}$ of a litre.

$$
\frac{1}{4} \text{ of a litre} = \frac{1}{4} \times 1000 \text{ ml} = 250 \text{ ml}
$$
$$
4 \text{ cups} = 4 \times 250 \text{ ml} = 1000 \text{ ml}
$$

$$
250 \text{ ml} < 1000 \text{ ml}
$$
So, $\frac{1}{4}$ of a litre $\boxed{<}$ 4 cups.

$$
4 \text{ cups} = 4 \times 250 \text{ ml} = 1000 \text{ ml}
$$
$$
1 \text{ litre} = 1000 \text{ ml}
$$

$$
1000 \text{ ml} = 1000 \text{ ml}
$$
So, 4 cups $\boxed{=}$ 1 litre.

$$
1 \text{ cup} = 250 \text{ ml}
$$
$$
\frac{1}{4} \text{ of a litre} = \frac{1}{4} \times 1000 \text{ ml} = 250 \text{ ml}
$$

$$
250 \text{ ml} = 250 \text{ ml}
$$
So, 1 cup $\boxed{=}$ $\frac{1}{4}$ of a litre.

$$
2 \text{ cups} = 2 \times 250 \text{ ml} = 500 \text{ ml}
$$

$$
500 \text{ ml} = 500 \text{ ml}
$$
So, 2 cups $\boxed{=}$ 500 ml.

$$
2 \text{ cups} = 2 \times 250 \text{ ml} = 500 \text{ ml}
$$
$$
\frac{1}{4} \text{ of a litre} = \frac{1}{4} \times 1000 \text{ ml} = 250 \text{ ml}
$$

$$
500 \text{ ml} > 250 \text{ ml}
$$
So, 2 cups $\boxed{>}$ $\frac{1}{4}$ of a litre.