Why is the answer a and not choice b?

5. Refer to the previous question. Suppose that the same poll was repeated in the United States (whose population is 10 times larger than Canada), but in this new pool, four times the number of people were interviewed. The resulting 95% confidence intervals will be: (a) about 1/2 as wide as the Canadian interval a (b) about 1/4 as wide as the Canadian interval (c) about 1/10 as wide as the Canadian interval (d) about 4/10 times as wide as the Canadian interval (e) the same size as the Canadian interval