Answer:
By definition of midpoint.
Step-by-step explanation:
proof:
1.Statement:
.
Reason: Given.
2.Statement:
is a right angle.
Reason:Given.
3.Statement:
.
Reason: By definition of right angle.
4.Statement:
.
Reason: Given.
5.Statement: 
Reason: Transitive property of equality.
6. Statement:
.
Reason: By definition of congruent triangles.
7.C is the midpoint of
.
Reason: Given.
8.Statement: C bisect 
Reason: By definition of midpoint.
9.Statement:
Reason: By definition of bisect.
10.Statement:
Reason: By reflexive property of congruence.
11.Statement:
.
Reason: SAS congruence postulate.