Let random variables X and Y be independent with the same variance sigma squared . Which one the following must be correct? a. V a r left parenthesis 0.5 X plus 0.5 Y right parenthesis equals sigma squared b. V a r left parenthesis X plus 0.5 Y right parenthesis equals 1.5 sigma squared c. V a r left parenthesis 0.5 X plus 0.5 Y right parenthesis equals 0.5 sigma squared d. V a r left parenthesis 0.5 X minus 0.5 Y right parenthesis equals 0