李老师，windows10上安装node报如下错误：
C:\Users\PC\projects\hello>node webServer.js
internal/modules/cjs/loader.js:638
throw err;
^

Error: Cannot find module 'express’
at Function.Module._resolveFilename (internal/modules/cjs/loader.js:636:15)
at Function.Module._load (internal/modules/cjs/loader.js:562:25)
at Module.require (internal/modules/cjs/loader.js:690:17)
at require (internal/modules/cjs/helpers.js:25:18)
at Object. (C:\Users\PC\projects\hello\webServer.js:7:15)
at Module._compile (internal/modules/cjs/loader.js:776:30)
at Object.Module._extensions…js (internal/modules/cjs/loader.js:787:10)
at Module.load (internal/modules/cjs/loader.js:653:32)
at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
at Function.Module._load (internal/modules/cjs/loader.js:585:3)
但是express已经导入了，这是怎么回事呢？