The cell phone is __________ useful invention of the last 50 years, however, some scientists __________ the disappearance of bees.

A. a; blame it for                                    B. an; blame it for

C. a; are blamed it for                                D. an; are blamed it for

This book shows the author's ________of learning.

[    ]

A. deep   B. deeply   C. depth   D. depths

 Thank you very much. It's ____ of you.
  A. kindest         B. a most kind         C. the most kind         D. most kind