Because it asks "By how many days will the length of the critical path change?" Not the length of the critical path.
The critical path is the shortest length of time to complete all tasks. Tasks can be completed simultaneously if their relative positions to one another logically allow this, and all tasks must be completed sequentially. To get the change in length, you first find the length of the original critical path (22 days), then adding the extra 7 days to Task EC, the new critical path length becomes 24 days, and so the difference is 2 days. Therefore the answer is B and not A.