Can you please post the link which has the explanation,Probably last time i missed it.
Don't know the post but, here is the solution:
from the stem , AB = OC = OB (Since OC and OB are radii of the same circle) --- (a)
from (1) , we have <COD = 60 . Let's solve:
Consider triangle CBO.
<CBO + <COB + <BCO = 180 (sum of angles in a triangle) --- (b)
<CBO = <BCO (since OB = OC, its an isosceles triangle)
equation "b" becomes:
2<CBO + <COB = 180
Now, <CBO = <BAO + <BOA (external angle in a triangle is equal to the sum of two internal angles)
also, <BAO = <BOA (from equation a above)
which implies, 4<BAO + <COB = 180 --> 4<BAO + (120 - <BOA) = 180
or, 4<BAO = 120 - <BOA = 180
or, 3<BAO = 60 (since <BAO = <BOA )
or, <BAO = 20
Hence (1) is sufficient.
Now from (2) we have, <BCO = 40 --> <CBO = 40 --> 2<BAO = 40 --> <BAO = 20. Hence (2) is sufficient.
I have not explained here but if you understood (1) above, I think this is easy to stand.
Therefore the answer is (D).