choose指一般意义上的选择.
例如：He advised farmers to choose the best seed-heads.他告诉农民要挑选最好的穗（留种）.
select指“精选”,即有所淘汰.
例如：He looked through the suits and selected the cheapest one for me.他细细地翻看这些衣服,给我挑了一件最便宜的.
pick out相当于“挑选出来”.并且,pick往往指比较容易的选择,但pick out常指费了心思或精力后才挑选出来的.
如:But by looking at the stages,we pick out a spacetime worm that makes up a person.
但看着这个阶段,我们联想到组成那个人的,时空虫.