For hundreds of years, letters were the only way to keep (3) ..................people who were any distance away and letter-writing was seen as an important skill for all learned people (4) .
For hundreds of years, letters were the only way to keep (3) ..................people who were any distance away and letter-writing was seen as an important skill for all learned people (4) .................
A. to be mastered
B. mastering
C. to master
D. mastered