Firm a Sells Lemonade and Firm B Sells Hot Chocolate

Firm A sells lemonade and firm B sells hot chocolate. If you invest $100 if firm A, in one year you will get back $(30 + T) where T is the average temperature (Fahrenheit) during the summer. If you invest $100 in firm B, in one year you will get back $(150 - T) , where T is the average temperature during the summer. The expected value of T is 70 and the standard deviation of T is 10. If you invest $50 in firm A and $50 in firm B, what is the standard deviation of your return on your investment?

A) 10
B) 20
C) 5
D) 0
E) None of the above.