Write the next sentence in this paragraph:

How to prune verbena
Wait until you start seeing new growth in the spring.
This will usually occur after the last frost. You may notice new green shoots along the base of the plant or leaves growing on the stalks.

pick from the following.
[a]. These are signs that the plant is ready for light pruning. If you notice new shoots or leaves growing on the branches, stop pruning the plants, especially when an increase of weather is expected in the future.
[b]. This is a sign that you should cut back. Trim old stems down to 2 inches (5.1 cm) above the ground.
[c]. Remove dead or diseased fronds when the pruning season is over. You should see shoots growing off the base of the plant or leaves growing on branches.
[d]. Blooming and flowering should start around july 4 to 6 months after your first frost. Do not attempt to prune verbena before the last frost.
[b].