Triangle PAB is formed by three tangents to circle with centre 
O and ∠APB = 40°, then angle 
AOB equals ______.
(A) 45°
(B) 50°
(C) 55°
  (D)     70°    
                                                                
    
 Triangle PAB is formed by three tangents to circle with centre 
O and ∠APB = 40°, then angle 
AOB equals ______.
(A) 45°
(B) 50°
(C) 55°
  (D)     70°