請問黃金第47篇Begging by Nestlings - 英檢

原文:
Paragraph 2：Further evidence for the costs of begging comes from a study of
differences in the begging calls of warbler species that nest on the ground
versus those that nest in the relative safety of trees. The young of
ground-nesting warblers produce begging cheeps of higher frequencies than do
their tree-nesting relatives. These higher-frequency sounds do not travel as
far, and so may better conceal the individuals producing them, who are
especially vulnerable to predators in their ground nests.
David Haskell created artificial nests with clay eggs and placed them on the
ground beside a tape recorder that played the begging calls of either
tree-nesting or of ground-nesting warblers. The eggs “advertised” by the
tree-nesters' begging calls were found bitten significantly more often than
the eggs associated with the ground-nesters' calls.

選項:
4. Paragraph 2 indicates that the begging calls of tree nesting warblers
a. put them at more risk than ground-nesting warblers experience
b. can be heard from a greater distance than those of ground-nesting warblers
c. are more likely to conceal the signaler than those of ground-nesting warblers
d. have higher frequencies than those of ground nesting warblers

正確選項是b，想請問為什麼答案是b呢?
麻煩各位的解答了^^"謝謝


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