7. [There can be one or more correct answers] if 21, ..., 250 are independent standard normal random variables, which ofthe following is/are TRUE: a} 13(21 +22 + ---+Zso) = 0 b) Var(21 + 22 + + Z50) = 1/50 c) 21 + Z; + + 250 follows (exactly) normal distribution d) 2% + 2% + + ZED follows a chi-squared distribution with df = 50 Your answer: is/are correct. [Note: you can briefly discuss your answers.]