What is the difference between 1+1 and 1：1 protection switching？（）
- A 1+1 is based on APS， while 1：1 is based on IPS.
- B 1：1 is used in SNCP switching， while 1+1 is used in MS-SPRing switching.
- C 1+1 sends signals on Working and Protect paths， while 1：1 sends signals only on Working path.
- D 1+1 sends signals on Working path ONLY， while 1：1 sends signals on both Working and Protect paths.