question on VRRP

Just curious how does following topology work with VRRP? 
Ra/Rb are two routers using VRRP; Sa/Sb are two switches; host a, b c. 
initially both a and b communites to c through Ra; then how does VRRP know if Sa failed, Rb will take over; if Sb failed, Ra will still carry the traffic. If Ra does not receive any ack from Rb, how can it figure out if it is Sa or Sb that has problem? Thanks, Ye. 
         c 
      /      \ 
    Ra      Rb 
    |         | 
    Sa----Sb 
     |\      /| 
     a b    a b         
    
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