Let X1, X2, X3 ~ Normal(0, 1). In each of the following cases, identify the distribution of the random variable with the value of the associated parameter (e.g. don't only say Normal, you need to specify the mean and variance). (a) (1 points) Z = X1 + 2X2 + 3X3. (b) (1 points) Z = X? + X3 + X3. (c) (2 points) Z = (X1 - X3)2/2. (d) (2 points) Z = X2+X2 2X2