Why do countries erect trade barriers that inhibit imports and exports

To protect their local markets and keep a health balance of payments.

Explanation:

They inhibit imports as those require forex and may hurt their own country's production and markets. They inhibit exports at times to prevent shortages of goods they require such as wheat when there is a drought or increases local demand.

answer; i believe the correct answer is (a) the correlation between the cause variable and outcome variable is zero;

classical republicanism, also known as civic humanism, is a form of republicanism  developed in the renaissance inspired by the governmental forms and writings of  classical  antiquity, especially such  classical  writers as aristotle, polybius, and cicero.