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阅读下列短文，从每题所给的四个选项(A、B、C和D)中，选出最佳选项。
Cool snowflakes
You have seen snow, but have you ever studied a snowflake? Have you looked hard at falling snowflake and seen its design up close?
It' s not easy to look at something so tiny．The flake often melts or blows away as soon t it lands．If looking at one seems difficult, imagine trying to photograph one．
But that' s what Ken Libbrecht has spent much of the past four years doing：catching snowflakes, putting them under a microscope and taking pictures of them．
Libbrecht is an American scientist who has written four books about snowflakes．Four of his snow photos will be put on US postage stamps this holiday season．
It' s the structure
In his latest book "Ken Libbrecht' s Field Guide to Snowflakes", he answers lots of questions about snowflakes in a way that' s easy to understand．
The beautiful structure of snowflakes is mainly the result of two things：conditions in the sky and the structure of water molecules(分子)．
A snowflake starts as a very small spot of dust．Water molecules in the air get attached to the dust, then freeze．These molecules are triangular．When enough of them become attached, a six-sided shape is formed．
Some snowflakes become flat and stay that way．Sometimes more water molecules freeze at the six corners and make the snowflake look like flowers．Other snowflakes turn into six-sided columns．
But if the changes in temperature and the amount of water in the air are just right, thin branches may grow on a baby snowflake as the snowflake travels in the air．The six-pointed starbursts(星放射状)are usually the prettiest．They are most likely to form when the weather is either just below freezing or around 5 degrees．
Be a snowflake watcher
1．A gentle snowfall is best．
2．Take a magnifying glass(放大镜)and a piece of black paper outside and wait a few minutes for the paper to get cold．
3．Hold the paper out flat until a few flakes have landed on it．Quickly examine them with the magnifying glass．Look for the best ones, or see how many types you can find．
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