he article helps me a lot understand the casper ffg.

While I am confused a little bit by that block C is finalized.

According to the paper “Casper the Friendly Finality Gadget” version 4, C is finalized if there is a supermajority link starts from C, but actually there is no supermajority link starts from C, since there are 60s arcs C->D, and 40s arcs C->E, none of which is more than 100*2/3=66.7 and therefore forms the supermajority link.

My understanding is that the votes can not be counted on vertices of the arcs alone(which is 100 on C indeed),but by the arcs(which is 60 at most).