7. [There can be one or more correct answers] If 21,..., 250 are independent standard normal random variables, which of the following is/are TRUE: a) E(21 +Zz+---+Zso) =0 b) Var(Z1 +Z2 + +250) = 1/50 c) 21 + 22 + + 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.]