The following data are for Problems $17$ through $19$: The correlation coefficients between several pairs of stocks are as follows: Corr $(A,B)=\mathrm{.}85$; Corr$(A,C)=\mathrm{.}60$;Corr$(A,D)=\mathrm{.}45.$ Each stock has an expected return of $8\%$ and a standard deviation of $20\%.$

$17.$ If your entire portfolio is now composed of stock A and you can add some of only one stock to your portfolio, would you choose $($explain your choice$)$

a$.$ B

b$.$ C

c$.$ D

d$.$ Need more data

$18.$ Would the answer to Problem $17$ change for more risk$-$averse or risk$-$tolerant investors? Explain.

$19.$ Suppose that in addition to investing in one more stock you can invest in T$-$bills as well. Would you change your answers to Problems $17$ and $18$ if the T$-$bill rate is $8\%?$