act of stretching or straightening out a flexed limb
[反]
flexion, flexure
a string of characters beginning with a period and followed by one to three letters; the optional second part of a PC computer filename
[同]
extension, file name extension, filename extension
[例]
most applications provide extensions for the files they create
most BASIC files use the filename extension .BAS
the most direct or specific meaning of a word or expression; the class of objects that an expression refers to
[同]
denotation, extension, reference
[例]
the extension of satellite of Mars' is the set containing only Demos and Phobos
the ability to raise the working leg high in the air
[例]
the dancer was praised for her uncanny extension
good extension comes from a combination of training and native ability
，可以收听真人发音 
小测验：选择正确的单词
