II. Consider the following relational schema for this section: R1(A1, A2, A3) R2(B1, B2) Domains of all attributes are STRINGs. Primary keys are underlined. Referential Integrity Constaints are represented by arrows. For each item below, specify whether it is true (T), or false(F). If it is true, shortly explain why. if it is false, shortly explain why. (a) A1 attribute is a Candidate Key of the R1 relation. - 1 point (b) B1 attribute is the Primary Key of the relation R2. - 1 point (c) if "d" is an A1 attribute value in R1, a tuple with B1 attribute value of "d" has to exist in R2. - 2 points