Determine whether the given description corresponds to an experiment or observational study.a. A stock analyst selects a stock from a group of twenty for investment by choosing the stock with the greatest earnings per share reported for the last quarter.b. A marketing firm does a survey to find out how many people use a product. Of the one hundred people contacted, fifteen said they use the product.c. A political pollster reports that his candidate has a 10% lead in the polls with 10% undecided.d. A doctor gives a new medication to half of his patients with the flu and a placebo to the other half of his patients with the flu.