Snail populations are dying off in certain European forests because acid

rain has depleted the soil of nutrients the snails need.  These forests are

the sole habitat of birds called passerines, which get most of the calcium

they need from snail shells.  When passerines get too little calcium, more

of their eggs have abnormally thin shells, and, accordingly, fewer off-

spring survive.  Therefore, acid rain will probably cause a decrease in

the population of European passerines.

 Which of the following, if true, most strengthens the argument given?

      A Nothing in the forests other than snail shells can provide passerines

  with calcium in the amount that they need. 

      B  Some cases of passerines laying eggs with very thin shells have

       been attributed to causes other than calcium deficiency. 

     C When the population of passerines in a forest declines, populations

        of predatory birds that prey on passerines also tend to decline in

       that forest. 

     D If the acidity of rain in Europe were reduced, then nutrients in forest

       soils would be restored to earlier levels. 

     E Because of recent expansion of certain European forests, the amount

       of habitat suitable for passerines has increased. 

答案A