A study on highway safety studied the driving habits of individuals. A sample of 74 cars travelling on the highway revealed an average speed of 102 kph with a standard deviation of 12.3 kph. Determine the critical value of the test statistic needed for a 95% confidence interval for the mean speed of all cars, if it is known that speed is normally distributed. Give the positive answer of the test statistic (z or t) exactly as found from tables (3 decimal places).