Which two of these statements correctly describe classic PIM-SM? (Choose two.)

A.
The lOS 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 lOS 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 pirn 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 pirn spt-threshold command to
“zero.”

Explanation:
They are checking you for syntax ip pim spt-threshold command to “infinity” is the right answer.
same source as above:
IP pim spt-threshold [vrf vrf-name] spt-threshold {kbps | infinity} [group-list access-list]
To configure when a Protocol Independent Multicast (PIM) leaf router should join the shortest path
source tree for the specified group infinity Causes all sources for the specified group to use the
shared tree.

http://www.cisco.com/en/US/docs/ios/12_2/ipmulti/command/reference/1rfmult2.html#wp1020111