Which of the following is NOT true about gamma?

a.	The magnitude of gamma is a measure of the curvature of the portfolio value as a function of the underlying asset price

b.	A long position in either a call or a put has a positive gamma

c.	A highly positive or highly negative value of gamma indicates that a portfolio needs frequent rebalancing to stay delta neutral

d.	A value of gamma close to zero indicates that a portfolio needs frequent rebalancing to stay delta neutral