Only this problem

Here is problem 14 for reference:

(8 p.) The semijoin of relations T and V, denoted TV, is the set of tuples of T that agree with at least one tuple of V on all attributes that are common to the schemas of T and V. Note that the schema of TV is the same as the schema of T. Consider the relations R(A,B,C,D) and S(B,D,E) given in Problem 14. Give four different expressions of relational algebra that are equivalent to RS (i.e. produce the same result including the same tuples with the same names of attributes). 4. (3 p.) What is the difference between the natural join RS and the theta-join RCS where the condition C is that R.A=S.A for each attribute A appearing in the schemas of both R and S ? For example, if the relations are R(A,B,C,D) and S(B,D,E), what is the difference