Louis, 55, and Rita, 52, have a large investment portfolio concentrated in individual stocks and stock mutual funds. They maintain cash reserves in a money market deposit account. Louis is employed as a senior nuclear engineer and earns an annual salary of $350,000. He is expecting to retire in five years. Select the investment that would be most suitable for the couple if they are seeking capital preservation and diversification. A) Biotechnology fund B) High-yield bond fund C) Tax-free municipal bond fund D) Special opportunities fund