如何在Ruby中释放用完的Excel自动化对象

require ‘win32ole‘

excel=WIN32OLE.new("excel.application")
wbk=excel.workbooks.add()
#some operate on wbk and excel...
#.
#.
#.
wbk.close
wbk=nil
excel.quit
excel=nil

#Garbage collect
GC.start