$(1)$ $Cl^-$ translocation is required during the transport of $CO_2$.
$(2)$ The probability of lung cancer increases due to smoking.

(A-D) $H_2O$ and $CO_2$ form $H_2CO_3$ in large amounts with the help of carbonic anhydrase,which dissociates into $H^+$ and $HCO_3^-$.
$(1)$ This $HCO_3^-$ moves out of the erythrocyte and reacts with $KHb$ to form $KHCO_3$,causing an ionic imbalance. If the osmotic pressure increases,water enters the erythrocytes,which may cause them to burst due to swelling.
To balance this,$Cl^-$ enters the erythrocyte from the blood plasma. This process is known as the chloride shift or Hamburger phenomenon. Thus,$Cl^-$ translocation is necessary to maintain the integrity of erythrocytes.
$(2)$ Smoking destroys the ciliated epithelium of the respiratory tract,which prevents the removal of dust particles and bacteria. Consequently,the accumulation of these particles and bacteria leads to uncontrolled cell division,inducing cancer. Therefore,smoking significantly increases the probability of lung cancer.