根据以下资料，回答10~13题。

We first think of the traditional or nuclear family．This is a two-generation family, the father and the mother and their own children．Most couples wanted to have four children, two boys, two girls.

Some nuclear families, however, may add one or more grandparents to come to live with them, that is three generations．This kind of family with grandparents, parents, and grandchildren is called an extended family．This family type was not very common during the later half of the twentieth century, but it's becoming more common now as an elderly grandparent moves in to live with a son or daughter．This is more possible now that American homes have become larger．What is interesting, however, is that after the grandchildren move out of the home and start their own families, this extended family shrinks back to a nuclear family, with just two generations again living together, a grandparent and parents, with the grandchildren coming only for occasional visits．

Now, the fatherless or motherless family is one kind of what we call a single-parent family．In the fatherless family it's just the mother and her children．As I said, this can be the result of the husband's death, of an unmarried mother, of a separation or divorce．There are also a growing number of motherless families--where the father raises the children, for any of the same reasons．A motherless family may also be fatherless, but still a family with one adult．This is becoming more common in the big cities where a grandmother will raise her daughter's children while the daughter goes elsewhere to work.

One other new kind of family is becoming increasingly more common．A single parent with one or more children will marry again．Perhaps the other parent is also a single parent．Together they will start what is called a blended family, which blends together or combines the children from two other families.

Who usually looks after the children in a both motherless and fatherless family in America?