After four years in America, he got __________.
A. a degree of doctor
B. a doctor's degree
C. the degree of a doctor
D. a doctor degree
The correct answer is B but I want to know why not C or A?
(Merriam Webster's Unabridged) Doctorate etymology: doctor: the degree, title, or rank of a doctor.
I have checked in dictionary that doctor cannot be used as an uncountable noun.