The times to pop a regular bag of microwave popcorn without burning it are Normally distributed with a mean time of 140 seconds and a standard deviation of 20 seconds. The times to pop a mini bag of microwave popcorn without burning it are Normally distributed with a mean time of 90 seconds and a standard deviation of 15 seconds. Suppose two independent random samples, 25 of each, are taken and the mean popping times are calculated. Let R = the popping time of a randomly selected regular-sized bag and M = the popping time of a mini-sized bag. Which of the following best describes the shape of the sampling distribution of x Overbar Subscript Upper R Baseline minus x Overbar Subscript Upper M? O Normal because both population distributions are Normal
O uniform because both sample sizes are less than 30
O skewed right because the difference in times cannot be negative
O skewed left because the sample sizes are less than 30 and the sampling variability is unknown