A．They are available at most country fairs．
B．They are taken in relatively high dosage．
C．They are collected or grown by farmers．
D．They are prescribed by trained practitioners．
A．They have often proved to be as helpful as doing mental exercise．
B．Taking them with other medications might entail unnecessary risks．
C．Their effect lasts only a short time．
D．Many have benefited from them．
A．How catastrophic natural disasters turn out to be to developing nations．
B．How the World Meteorological Organization studies natural disasters．
C．How powerless humans appear to be in face of natural disasters．
D．How the negative impacts of natural disasters can be reduced．