Assume that the function call operator() is overloaded for data type String in the usual sense of selecting a substring from a larger string. For a String object string1 with the character string "ABCDEFGHI", what string does string1( 4 , 2 ) return?

a. "EF"
b. "EFGHI"
c. "CDEF"
d. "CD"

a. "EF"