Which one of the following statement is true?

a$.$ Systmatic risk affects a large number of assets or the aggregate market an it is measured by beta

b$.$ The risk$-$free market return is usually measured by the return on the treasury bill or t$-$bill

c$.$ Diversifation refers to the practice of investing in a variety of diverse assets as a mean of reducing risk

d$.$ A portfolio could be described as a group of assets by an investor

e$.$ The higher the standard deviation of returns on a security, the lower the security$$s risk and therefore the lower its expected rate of return