i have a 2005 c240 4matic and it has only 74000 miles. when i start the car, a large cloud of white smoke comes out the exhaust but goes away after a short while . What is it caused by ?

Well, sometimes it does that when it cold or has impurities when sitting. Although, if you smell coolant in the engine or your spark plugs are clean with no carbon, then you have a coolant leak or a head gasket.

I would check the oil dipstick and smell it and look to see if there isn't milking or smell like antifreeze.