Thr ar two main causs of cancr: hrdity and nvironmnt. W can do nothing to chang our hrdity, that is, th basic physical charactristics that wr passd on to us by our parnts. Howvr, w can do a grat dal to control our nvironmnt, which may account for 80-90 %of all cancrs. nvironmntal pollution can b dividd into two kinds: prsonal and imprsonal. Prsonal pollution may b dfind roughly as unhalthy habits such as smoking, drinking, and ating th wrong foods. Clarly, with nough will powr w can control this prsonal nvironmnt. Imprsonal pollution, on th othr hand, rfrs to thos things which ar byond our individual control. On xampl would b industrial pollution, which is vry hard to control bcaus of th xpns involvd. It would b xpnsiv in trms of profits and prhaps jobs. Th Amrican automobil industry, for xampl, rsistd th law rquiring smog-control dvics bcaus it fard th addd xpns would rduc profits. Nvrthlss, som laws protcting th nvironmnt hav bn quit succssful in rducing pollution.