The following sentence has been divided into parts. One of them may contain an error. Select the part that contains the error from the given options. If you don't find any error, mark 'No error' as your answer.
By the time she retires, / she has been teaching / at the university for 30 years.
1
she has been teaching
2
at the university for 30 years
3
By the time she retires
4
No error