资料题：According to the passage a towboat is used to（　）。

What if you were in a boat that stopped running? You might be far from land.How would you get back? You could call for a towboat.Towboats are small boats that move bigger boats.People on towboats can tie a line to a big boat.Then the towboats can pull from the front or sides.Or they can push from the back.Sometimes they don't have to move the big boat at al.They might just bring someone to fix it.After it is working again, the big boat can go on its way.There are flat boats that can't move without towboats pushing them.These fat boats carry goods over the water.They are very big.So they can hold a lot of things.Same flat boats carny wood.It comes from trees.People cut these down and saw therm into logs.Then they put the wood on a flat boat.That:'e where the towboat comes in.It pushes the flat boat along.This is how the wood gets to a city that needs it.People who run boats are called captains.Captains of towboats know how to do many things.They have to get very close to a big boat to tie a line to it.At the same time, they must take care not to run into it.And they must know their way around the sea.Rocks may be just under the water.Captains must know how to keep their boats away from them.