Tom's portfolio consists solely of an investment in Merck stock. Merck has an expected return of 13% and a volatility of 25%. The market portfolio has an expected return of 12% and a volatility of 18%. The risk-free rate is 4%. Assume that the CAPM assumptions hold in the market.
Assuming that Tom wants to maintain the current expected return on his portfolio, then the amount that Tom should invest in the market portfolio to minimize his volatility is closest to:
A: 125%
B: 100%
C: 88.8%
D: 112.5%