One of the biggest problems with telephone surveys is that 
            a.  it takes too long to collect the results.    b.  they're too impersonal.    c.  the cost is higher than other methods.    d.  people often refuse to participate..
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One of the biggest problems with telephone surveys is that people often refuse to participate. - D.

D. people often refuse to participate.

Explanation:

One of the biggest problems with telephone surveys is that people often refuse to participate. Many people ignore the calls or block the caller because they are not interested or refuse.

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