[metapost] Can't label with "btex etex"

[3160105656 at zju.edu.cn]

Hi.I am a user of MetaPost.

Today I failed to compile a mp file I wrote(example.mp and I put it in accessory).

The error is cause by the statement "label(btex 1 etex,(0,0));" which I have noted it.

And my compiler told me that it "Unable to read  mpx file". 

I want to know how to solve the problem for I really need to label my figure.Thanks!
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