Read each sentence to find out whether there is any grammatical error in it. The error, if any will be in one part of the sentence. The letter of that part is the answer. If there is no error, the answer is 'E'. (Ignore the errors of punctuation, if any).
This book is designed to increase a working knowledge of spoken and written English.
This book is
designed to increase
a working knowledge of
spoken and written English.
No error
Correct answer is C
the working knowledge of