The sales data for January and February of a frozen yogurt shop are approximately normal.  The mean daily sales for January was $300 with a standard deviation of $20.  On the 15th of January, the shop sold $310 of yogurt.
The mean daily sales for February was $320 with a standard deviation of $50. On the 15th of February, the shop sold $340 of yogurt.
Which month had a higher z-score for sales on the 15th, and what is the value of that z-score?

a.)  January, with a z-score of 0.5.
b.)  February, with a z-score of 1.
c.)  February, with a z-score of 0.4.
d.)  January, with a z-score of 0.2.