You are asked to study the causal effect of hours spent on employee training​ (measured in hours per worker per​ week) in a manufacturing plant on the productivity of its workers​ (output per worker per​ hour). Write in the box the option letter ​(a, b, c or​ d) on the right that best describes the statement on the left. Choose a random group of employees to receive ten hours per week in additional training for a period of four weeks.​ Then, estimate the difference in productivity between workers who received the additional training and those that did not