Which of the following statements is true about the hedge ratio?

a. It is also called the option's gamma.

b. The hedge ratio of a put option must be negative and smaller than 1 in absolute value.

c. It is the change in the option price and for a $1 change in the volatility of the underlying asset.

d. The hedge ratio of a call option must be negative and smaller than 1 in absolute value.

e. It is the change in the option price for a $1 change in the strike price.