Which sentence has a formal style and tone?
a) it ain't necessarily true that people who are good at geometry often turn into engineers.
b) see, the problem is, you guys just don't know what is causing the water shortage in this city.
c) y'all need to study much with much more diligence, if y'all are going to do well on this next test.
d) scientists have spent years trying to determine how tsunamis can be predicted with greater accuracy.