Suppose we have the following code to call a function foo: movi r12, 4 movi movi r20, 0x5555 r18, 6 call foo add r22, r12, r18 r22, r22, r20 xor Which registers will we have to save before the call foo instruction? Select one or more: O a. r18 b. r22 c. r20 O d. r12