Present Passive versus Past Passive

(TESOL Worksheet--Present Passive, Past Passive)
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Read these sentences.  What is the difference between them.

A	B
Mice are usually caught by cats.	Yesterday the mouse was caught by a cat.
Expensive cars are always driven by rich people.	This car was driven by a rich man.
Picnics are ruined by rain.	Our picnic yesterday was ruined by the rain.
Cats are chased by dogs.	Our cat was chased all over town by a dog yesterday.
Books are written by writers.	Dracula was written by Bram Stoker.
Students are taught by teachers.	I was taught by a very good teacher last term.
Chocolate is eaten by people.	All my chocolate was eaten by my brother.
Floods are caused by rain.	The flood last week was caused by heavy rains.
What is the difference between column A and column B?