Newborn babies average 7.8 pounds with a standard deviation of 2.1 pounds. A random sample of 30 babies born to mothers who took an anxiety medication during pregnancy shows a mean birth weight of 7.2 pounds. What's the probability of a sample mean of 7.2 pounds or less?
a) 0.1587
b) 0.2257
c) 0.8413
d) 0.7743