Some Mammals Give Birth to One Offspring at a Time

Some mammals give birth to one offspring at a time, and others to larger litters. Since more offspring per birth could mean higher reproductive success, why doesn't natural selection favor large litters in all mammals?

A) Natural selection does this; most mammals will eventually evolve the ability to have multiple offspring per litter.
B) The number of offspring per litter has no effect on reproductive success because in most cases only one survives anyway.
C) Reproducing involves a number of trade-offs, including quantity (number) versus quality of offspring. Larger litters mean lower-quality individual offspring.
D) It is not possible to change the number of offspring per litter in mammals; all mammals have about the same number of offspring.