28. 다음 글의 밑줄 친 부분 중, 어법상 틀린 것은? [3점]
Psychologists who study giving behavior ① have noticed
that some people give substantial amounts to one or two
charities, while others give small amounts to many charities.
Those who donate to one or two charities seek evidence about
what the charity is doing and ② what it is really having a
positive impact. If the evidence indicates that the charity is
really helping others, they make a substantial donation. Those
who give small amounts to many charities are not so
interested in whether what they are ③ doing helps others ─
psychologists call them warm glow givers. Knowing that they
are giving makes ④ them feel good, regardless of the impact
of their donation. In many cases the donation is so small ─
$10 or less ─ that if they stopped ⑤ to think, they would
realize that the cost of processing the donation is likely to
exceed any benefit it brings to the charity. (149 words)