[plt-scheme] Collect-Garbage not enabled on Dr. Scheme?

Hi All --

After creating function 'create-tests,' I ran the following per HtDP
exercise 26.3.6 --

(define test-case (create-tests 10000))
(collect-garbage)
(time (sort test-case))
(collect-garbage)
(time (quick-sort test-case))

-- which returned this:

collect-garbage: name is not defined, not a parameter, and not a
primitive name

I tried changing the teaching language to Advanced Student and still
received the same error message.  Is there something I need to do to
enable the collect-garbage function?  Also, what does collect-garbage
do?

Thanks!

Dave Yrueta