A、The IOS default is for a last-hop router to trigger a switch to the shortest path tree as soon as a new source is detected on the shared tree.

B、The IOS default is for every one of the routers on the shared tree to trigger a switch to the shortest path tree as soon as a new source is detected on the shared tree.

C、The default behavior of switching to the shortest path tree as soon as a new source is detected on the shared tree can be disabled by setting the value in the ip pim spt-threshold command to "infinity."

D、The default behavior of switching to the shortest path tree as soon as a new source is detected on the shared tree can be disabled by setting the value in the ip pim spt-threshold command to "zero."