(a) (5 points) What is a random variable? (b) (8 points) What is the difference between the unconditional mean and the conditional mean of a random variable? (c) (5 points) Suppose that X and Y are random variables, and let cov (X, U) represent their covariance, and px,y represent their linear correlation. What's the relationship between cov (X, U) and px,y? (d) (5 poitns) Suppose that the correlation between two random variables is zero. Does this mean there is no relationship between these variables? In other words, does this mean these variables are independent? (e) (15 points) What are they key components of the classical linear regression model? Describe the role that each component plays. (f) (6 points) What is the difference between o and Bo? (g) (6 points) What's the difference between a linear and a non-linear regression model?