The correct answer is: B. RPQS
The first sentence, S1, introduces the topic of the passage, which is the art of growing old. The second sentence, P, states one of the two dangers to be guarded against in old age, which is undue absorption in the past. The third sentence, Q, states the opposite of P, which is that one’s thought must be directed to the future. The fourth sentence, R, provides a reason for Q, which is that there is something to be done in the future. The fifth sentence, S, provides an example of something that one should not do in old age, which is to live in memories. The sixth sentence, S6, provides a reason for S, which is that one’s own past is gradually increasing weight.
The other options are incorrect because they do not follow the logical order of the sentences. For example, option A, QSRP, puts sentence Q before sentence P, which is illogical
because Q is a consequence of P. Option C, RPSQ, puts sentence R before sentence P, which is also illogical because R is a reason for Q. Option D, QPRS, puts sentence Q before sentence S, which is illogical because Q is a consequence of S.