There are two boxes marked A and B. Box A contains 4 red and 3 white balls, and Box B contains 3 red and 4 white balls. A box is selected at random and a ball is taken from it. Probability that the ball is white is
(a) $\frac{1}{2}$
(b) $\frac{12}{49}$
(c) $\frac{37}{49}$
(d) $\frac{1}{4}$