If string s1 has the value "computer" and string s2 has the value "promise", which call to insert will produce the string "compromise"?

a. s1.insert( 4, s2, 0, string::npos );
b. s1.insert( string::npos, s2, 0, 4 );
c. s2.insert( 0, s1, 0, 3 );
d. s2.insert( 3, s1, 0, 3 );

c. s2.insert( 0, s1, 0, 3 );