Suppose we are adding up all the values in an array called a$,$ using a for loop with loop index k$.$ Which statement, if placed in the body of the loop, will correctly add the current element to the sum?

a$.$

sum $+=$ a$[$ k $]$;

b$.$

sum $=$ a$[$ k $]$;

c$.$

sum $+=$ k;

d$.$

sum $=$ k;

e$.$

sum $+=$ k$[$ a $]$;