Need answer and explanation.

Problem 2. Let's get more oomfortahle with randomization based calculations. a) Consider a treatment-control experiment with N unite where for each unit 1' we observe a pre- treatment {i.e., not affected by treatment} eovariate Xi. Let if!\" he the observed eovariate mean for the treatment group and I?\" he the observed eovariate mean for the eontrol group. Prove that under a complete randomization with :11 units assigned to treatment and on unite assigned to control, these means are equal in expectation. That is... prove that EL? 11\"] = EPI'EM]