Solve the problem.
In a Psychology class, the students were tested at the end of the course on a final exam. Then they were retested with an equivalent test at subsequent time intervals. Their average scores after t months are given in the table.

Using a graphing utility, fit a logarithmic function y = a + b ln x to the data. Using the function you found, estimate how long will it take for the test scores to fall below 84%. Express your answer to the nearest month.
◦ 8 months
◦ 20 months
◦ 10 months
◦ 12 months