Tariffs frequently impact the flow of goods internationally. In the section, you will draw on data from one of these tariffs, investigating how these disruptions impact exports. For the next three questions, you will use the data posted on Canvas as trade policy.csv. This data contains several variables: post is a binary variable indicating the tariff has been introduced (post = 1, pre = 0); treat is a binary variable that equals 1 if a country is "impacted" by the tariff barrier (i.e., a regulation increasing the cost of exports) and 0 if a country is "not impacted"; air conditioners pc is the number of air conditioners exported per capita by a given country in a year. 1. Use a difference-in-differences design to estimate the impact of the tariff on exports of air conditioners. Regress air conditioners pc on post, treat, and the interaction of post and treat. Which coefficient reveals the change in exports after the tariff was introduced? Explain and interpret the result. 2. The difference-in-differences design is valid when treatment and control have common trends. What is a common trend? Write down the definition. Next, give an example (about this case) when this assumption likely violated