which bike to buy?

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I decided to start biking to work everyday, but I haven't been biking for a long time. My only cardio workout is on an ellipitical and I only used as a warmup for my workout routine.

I'm not really looking for an expensive bike...like mayb around $200-300 CAD?
I didn't know there were so many kinds of bikes. I'm not sure if I need a hybrid or a moutain,but I've been checking it on canadiantire.ca.

So what kind of bike would be at my price range and what type of bike do I need? The distance I believe is around 8 miles or 15 km.

I'm planning to get this bike I saw someone selling for $80 CAD. What do guys think about it?

void 21 speed, mens, dual shocks, grip shift...............$80
http://img33.imagevenue.com/img.php...ee_HPIM0295.JPG
http://img12.imagevenue.com/img.php...af_HPIM0296.JPG
http://img22.imagevenue.com/img.php...77_HPIM0297.JPG
http://img39.imagevenue.com/img.php...63_HPIM0301.JPG

ccm 18/21 speed, mens, excellent tires, oversized tubing, verrry good condition..........$80
http://img29.imagevenue.com/img.php...3a_HPIM0298.JPG
http://img13.imagevenue.com/img.php...10_HPIM0299.JPG
http://img36.imagevenue.com/img.php...41_HPIM0300.JPG

I'm not planning to do long term just mayb for a month or 2 since I've hit some problems with my car for now and thought it would be good to start doing some cardio