$1.$ A portfolio is $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_.$

A$)$ a group of assets, such as stocks and bonds, held as a collective unit by an investor B$)$ the expected return on a risky asset

C$)$ the expected return on a collection of risky assets

D$)$ the variance of returns for a risky asset

E$)$ the standard deviation of returns for a collection of risky assets