S1:tThe heart is pump of life.P :tThey have even succeeded in heart transplants.Q :tNowadays surgeons are able to stop a patients heart and carry out complicated operations.R :tA few years ago it was impossible to operate on a patient whose heart was not working properly.S :tIf heart stops we die in about five minutes.S6:tAll this was made possible by the invention of heart-lung machine.The Proper sequence should be:

• A. SRQP
• B. SPRQ
• C. SQPR
• D. SRPQ