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

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