The Heights of a Simple Random Sample of 400 Male $\sigma$

The heights of a simple random sample of 400 male high school sophomores in a Midwestern state are measured.The sample mean is = 66.2 inches.Suppose that the heights of male high school sophomores follow a Normal distribution with a standard deviation of $\sigma$ = 4.1 inches.Suppose the heights of a simple random sample of 100 male sophomores were measured rather than 400.Which of the following statements is true?

A) The margin of error for the 95% confidence interval would increase.
B) The margin of error for the 95% confidence interval would decrease.
C) The margin of error for the 95% confidence interval would stay the same because the level of confidence has not changed.
D) The standard deviation $\sigma$ would decrease.