18. Four children were born in the city hospital yesterday. Assuming the probability of having a boy or a girl is equal, which of the following statements is the most likely.
(A) all boys
(B) all girls
(C) two boys, two girls
(D) three of one gender, one of the other
(E) all probabilities are equal
18. D.

The probability of all being boys or all being girls is
$$
\left(\frac{1}{2}\right)^{4}=\frac{1}{16} \text {, }
$$

The probability of two boys and two girls is
$$
\mathrm{C}_{4}^{2}\left(\frac{1}{2}\right)^{2}\left(\frac{1}{2}\right)^{2}=\frac{3}{8} \text {, }
$$

The probability of one girl and three boys or three girls and one boy is
$$
\mathrm{C}_{4}^{1}\left(\frac{1}{2}\right)^{1}\left(\frac{1}{2}\right)^{3}+\mathrm{C}_{4}^{3}\left(\frac{1}{2}\right)^{3}\left(\frac{1}{2}\right)^{1}=\frac{1}{2} .
$$