Which answer about the "sharp ratio" is true ?

a. This is the ratio of the average return and volatility of historical returns. We often use the annualised version of these statistics. Ideally, we would use the average excess return, however in recent years, the risk-free rate is virtually zero

b.This is the ratio of the mean and volatility of returns when we assume a risk-free rate equal to zero

c.The simplified Sharpe Ratio is, in other words, what most investors call the Sortino Ratio

d.This is the ratio of the average excess return and volatility of historical returns. We often use the annualised version of these statistics. We could omit the risk-free rate since, in recent years, it is virtually zero