I am new in Mongodb.

I want to cluster mongodb by using replica set. I have created three replicaset rs0,rs1,rs2. and run db parallel and no issue. But when i have close any two instance one primary and secondary then single instance does't make as primary it remain secondry

 Step1 := 
   rs0: primary 
rs1:secondary
rs2:secondary

Every thing is ok. When

  Step2 :- rs0: closed
rs1:closed
rs2:secondry  (This does't make as primary)

Why???