Write the next sentence in this paragraph:

How to construct a sandstone wall 
Pick the location. 
Make sure you aren't preventing natural run-off water from the position of the wall and footing. Observe the land in question for a few days prior to doing any work.

Available choices:
[a]. Trace the run-off the day before the project, take your measurements and cut out a path to the location. The path may be difficult to follow, but be sure to have all your permits completed in two days.;
[b]. Pay special attention to the area after it rains. You don't want to disrupt the flow of water so that it floods into your neighbor's property.;
[c]. Select a location that has not been disturbed in a long time. The distance of your yard from your home is always a good place to begin for any repairs you might have to make.;
[d]. After the initial survey, use this as a basis for final calculations. Later, you can calculate your actual cost.;
[b].