1. A survey was conducted to compare how long (in seconds) five swimmers could hold their breath underwater before and after warm-up. The researcher thought that the mean difference between before and after would be zero. The collected data is shown in the following table. Before After 26 21 30 28 22 21 23 25 45 43 (a) Calculate the mean difference between before and after warm-up. (5 ) (b) Calculate the standard deviation difference between before and after warm-up. (5 ) (c) Test the researcher hypothesis at 5% level of significant. ( 10 )