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 postiions for extended time periods.

c) There is no difference in the risk an investor takes if he buys stock on the margin or if he pays for the entire purchase in cash.

d) If $1,000 worth of stock is bought on 70% margin, the buyer must invest $300 in cash.