The scatterplot above shows the number of visitors to a railroad museum in Pennsylvania each year from 1968 to 1980, where $t$ represents the number of years since 1968 and $n$ represents the number of visitors. The x-axis shows the years since 1968, and the y-axis shows the number of annual visitors. A line of best fit is also shown. Based on the line of best fit shown, which of the following is the best estimate for the number of visitors in 1975?