Which of the following statements about margin trading is correct? a. When you trade on margin, the equity in your accounts acts as collateral for a brokerage-provided loan b. Investors usually remain in margin positions for extended time periods c. There is no difference in the risk an investor takes if he buys stock on margin or if he pays the entire purchase in cash d. If $1,000 worth of stock is bought on 70% margin, the buyer must invest $300 cash.