和老师写的代码一样，但是一运行就报以下错误，为什么啊！

Error: No default engine was specified and no extension was provided.

   at new View (e:\imooc\node_modules\express\lib\view.js:62:11)

   at EventEmitter.render (e:\imooc\node_modules\express\lib\application.js:569:12)

   at ServerResponse.render (e:\imooc\node_modules\express\lib\response.js:961:7)

   at e:\imooc\app.js:16:9

   at Layer.handle [as handle_request] (e:\imooc\node_modules\express\lib\router\layer.js:95:5)

   at next (e:\imooc\node_modules\express\lib\router\route.js:131:13)

   at Route.dispatch (e:\imooc\node_modules\express\lib\router\route.js:112:3)

   at Layer.handle [as handle_request] (e:\imooc\node_modules\express\lib\router\layer.js:95:5)

   at e:\imooc\node_modules\express\lib\router\index.js:277:22

   at Function.process_params (e:\imooc\node_modules\express\lib\router\index.js:330:12)