题目10单选题
设关系R(A,B,C)和S(B,C,D),下列各关系代数表达式不成立的是( )A. <img src="https://tihai-oss-cloud.itihey.com/img/73eebb02bec7b12fe697e566d880a875.png">B. <img src="https://tihai-oss-cloud.itihey.com/img/91830563936a84e32a7aa657416f3105.png">C. <img src="https://tihai-oss-cloud.itihey.com/img/fd75b5b6d960f89889b79706f130333c.png">D. <img src="https://tihai-oss-cloud.itihey.com/img/8c729447fe301c13167d2c9357685dd7.png">