Which of the following letter clusters will replace the blanks in the given series? stss _ ssstsss _ tss _ _ stsssssst

Find the missing group of letters in the given sequence: QAR, RAS, SAT, TAU, ____

In the given series, choose the missing term from the given alternatives: DEF, HIJ, MNO, ?

Select the letter-cluster from among the given options that can replace the question mark (?) in the following series. ANZ, BOY, DQW, GTT, ?