7. Assume the following causal model for the grades in an exam: $ = % + !$ + " $ + -$ $ + $ Where Nervous is a dummy that equals 1 if the student is nervous during the exam, and 0 otherwise. Prepared is a dummy that equals 1 if the student is well prepared for the exam, and 0 otherwise. You may assume that [$|$, $] = 0 Interpret in words each of the coefficients. What are the expected signs for each of them? Justify your