single entry visa and layover

Hi, I am planning to travel throughout China in September so I am about to apply for my tourist L.
However, In July, I am traveling to Canada from Seoul with a 20 layover in Guangzhou . I am worried that my single entry will be used up on this layover if I decide to leave the airport. Can I choose the free transit when at the airport and thus keep my single entry intact for my vacation in September??