In the third paragraph, the word "background" means " ______".

A．has nothing to do with heredity
B．is not known by scientists
C．has little to do with the working of their brains
D．is that left-handers are mainly twins

A．If both parents are left-handed, about 50% of their children will be so.
B．The greater part of left-handers have right-handed parents.
C．One in ten of Americans are left-handed.
D．Most of us will favor the right.