How many of the Americans are left-handers（）

A．We now fully understand how risk factors trigger heart attacks.
B．We recently began to study how risk factors trigger heart attacks.
C．We have not identified many risk factors associated with heart attacks.
D．We do not fully understand how risk factors trigger heart attacks.

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.