13．
The Smallest Life Around Us
Tiny plants and animals live all around us．We cannot see them because they are so small，but they are everywhere．
They live in the soil，the air，and in rivers and oceans．They grow on mountains and in caves．Some live in the ice at the north and south poles； some live where it is very hot．They are found throughout cities and in the country．They grow on other plants and animals．Some are even living on you，on your skin，hair，and inside your body，in your mouth，nose，and all through your intestines（肠）．
These tiny plants and animals are called microbes．The word microbe is made from two other words：micro，meaning small，and bios，meaning life．So，microbe means small life．
Microbes have been on earth for a very lon tme．They may have been the first forms of life to be alive．Today there are more microbes in the world than all other living things．
People did not know microbes had existed（存在） long．They were too small for them to see．
But people did see many changes in their world-important changes．And they did not know what caused them．They saw food that they kept too long change color and bein to smell bad．They saw cuts and wounds grow red and painful and become infected（感染）．They saw old plants and leaves and trees crumble apart on the forest floor and disappear into the soil．Such changes were a natural part of the world，but people did not understand them．They did not know these changes were evidence of tiny plants and animals growing all around them
31．Why couldn't people see microbes？A
A．Because they are too small．
B．Because they live inside our body．
C．Because they live far from humans．
D．Because they may have been the first forms of life to be alive．
32．The underlined phrase"crumble apart"in the last paragraph means"C"．
A．grow taller
B．change into soil
C．break into tiny pieces
D．become colourful and energetic
33．Choose the answer that completes the web below．
A．Microbes are very small
B．Microbes are all around us
C．People can't see microbes
D．Food begins to smell bad．
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