Why might a baby be born early if the mother smokes?

Smoking during pregnancy has a number of problems for the mother and the baby. Often, smoking during pregnancy leads to miscarriage or premature birth of the baby. The weight of the baby is also lower as compared to the babies whose mothers do not smoke during pregnancy. As the conditions inside the mother are not safe of the mother smokes, hence babies are often born early for such mothers. The placental might detach early from the uterine tube.  

helicase, i think thats it

b is the correct answer