7 var occurs in a loop of computer programm.The variable nd step during which they must be stored are
t: step 1 through 6;u:step2;:steps 2 through 4 ;w:step 1 ,3 and 5;x:step 1 and 6;y:step3 through 6; nd z:step 4 nd 5. How many different index registers are needed to store these variables during execution?