2.  Which of the following statements are true about a call to <tt>free</tt>?
None may be true, or more than one statement may be true.
Briefly explain your
answer.
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a.  A call to <tt>free</tt> always increases the number of free blocks.
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b.  A call to <tt>free</tt> may leave the number of free blocks unchanged.
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c.  A call to <tt>free</tt> may reduce the number of free blocks by 1,
but never by more than 1.
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d.  A call to <tt>free</tt> may reduce the number of free blocks by 2 or more.