ruby "instance" WTF

main.rb

def showA0()
 puts self.class
 puts @a
end

class B
 def showA(t)
  @a = t
  showA0() 
 end
end
  
class C
 def showA(b, t)
  @a = t
  b.showA(t + 1) 
  showA0()
 end
end

class D
 def showA(b, t)
  b.showA(t + 1) 
  showA0() 
 end
end

b = B.new
c = C.new
d = D.new
b.showA(10) # B, 10
b.showA(5) # B, 5
c.showA(b, 15) # B, 16, C, 15
d.showA(b, 15) # B, 16, D, nil