This is one of the most important inventions of this century.

A. has done B. did C. does D. had done E. No correction required

A. You don't need to come unless you want to B. You come only when you want to C. You come unless you don't want to D. You needn't come until you don't want to E. No correction required

A. initials B. signatures C. pictures D. middle names E. No correction required

A. to whom I sell B. to who I sell C. who was sold to D. to whom I sold E. No correction required