Snail populations are dying off in certain European forests because acid # `+ ?: w! }" H. S) B- s4 [* O- x' D
rain has depleted the soil of nutrients the snails need. These forests are
( B, c0 K5 r" a9 m6 ~$ w/ i- bthe sole habitat of birds called passerines, which get most of the calcium . m1 }! \4 z6 y+ w6 W. Y
they need from snail shells. When passerines get too little calcium, more # a3 h/ f0 q1 X' R6 P5 E4 y
of their eggs have abnormally thin shells, and, accordingly, fewer off-/ {" t& c: i8 Y1 y4 d8 B
spring survive. Therefore, acid rain will probably cause a decrease in 1 Q4 m% r2 d6 B6 I: i4 d0 h9 D
the population of European passerines.
0 T4 V( f: S$ O% V* T9 S# ^- B; d6 {/ _9 c Which of the following, if true, most strengthens the argument given?
6 W' I) X4 D. q+ [: G) J; c' @ A Nothing in the forests other than snail shells can provide passerines
& w8 z$ s) A3 F, M! U: Y with calcium in the amount that they need.
8 p) _& J; C" j+ u B Some cases of passerines laying eggs with very thin shells have # o2 s/ G; _' D H% e; y1 q
been attributed to causes other than calcium deficiency. ( r, N) u' f) o' G
C When the population of passerines in a forest declines, populations ( q" E( e( J- z5 ~9 ^) ]& v8 B
of predatory birds that prey on passerines also tend to decline in
2 v& { t, B: {/ L that forest. + d) ~" U( [- m
D If the acidity of rain in Europe were reduced, then nutrients in forest
# a" a7 r6 o2 A9 w O& o, r soils would be restored to earlier levels.
5 C* c. T; E$ @4 _: F9 E! v E Because of recent expansion of certain European forests, the amount
& r$ ^* J* P8 f of habitat suitable for passerines has increased.
# n2 b) a8 q8 E! f" r答案A |