2. Citibikes were introduced in New York City in 2013. At first, they were only south of 59th street in Manhattan, now they are available throughout Man- hattan, and many parts of Brooklyn, Queens, and the Bronx. Suppose you are interested in the impact of Citibike on the demand for the subway. You have data on monthly usage of every subway station in the city from 2010 until the present. You also have the following variables: usage: Monthly subway usage for that particular station Ever Citibike: At any point from 2010 to the present, has there been a Citibike station within .1 miles from the station. Citibike: In the current month (the month in which usage is determined), is there a Citibike station within .1 miles from the station (a) Specify a difference-in-differences regression (b) Which coefficient would be your estimate of the causal effect of Citibike on subway ridership? (c) In 2020, the pandemic began, which dramatically lowered subway us- age everywhere in the city. Would that undermine your difference-in- differences estimate? Why or why not?