Which two of these statements correctly describe classic PIM-SM？（）
- AThe 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.
- BThe 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.
- CThe 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."
- DThe 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."