Babies Born with Low Birth Weights (Less Than 2500 Grams)

Babies born with low birth weights (less than 2500 grams) are at an increased risk for many infant diseases. Researchers in North Carolina collected data to see what variables may influence the birth weight (in grams) of a child, including whether the mother drank alcohol during pregnancy, whether the mother smoked during pregnancy, the mother's age (years) , the gestation of the pregnancy (number of weeks from conception until birth) , the mother's race, the length of the birth (hours) , and several others. Is this a randomized experiment or an observational study?

A) Randomized experiment, since researchers measured more than two variables.
B) Observational study, since birth weight cannot be randomly assigned.
C) Observational study, since whether the mother drank alcohol during pregnancy was not randomly assigned.
D) Observational study, since this was not a random sample.