The speed of pickup of ridesharing services like Uber and lift seems to have surpassed that of ambulance services. The main response time of ambulances across the United States is 15.3 minutes with a standard deviation of 12.8 minutes. For rides sharing services, the mean pick up time across United States is 8 minutes with the standard deviation of 5.2 minutes. Based on these estimates, which of the following gives the standard deviation of the sampling distribution of the difference in sample means for samples of 30 ambulance rides and 40 ridesharing rides (ambulance - ride sharing)?
A) 12.8/sqt.30 + 5.2/sqt.40
B) 12.8/sqt.30 - 5.2/sqt.40
C) sqt. 12.8^2/30 + 5.2^2/40
D) sqt. 12.8^2/30 - 5.2^2/40