Consider the following eight phrases: 1. careful inventory and mapping to determine dangers 2. weathering of minerals to produce clay 3. controlled blasting of unstable slopes 4. controlled water drainage 5. retaining walls along highway embankments 6. spraying shotcrete on road cuts 7. removing support at the toe of the slope 8. adding weight at the top of the slope which of these are factors that lead to mass movement? choose one:
a. choices 1, 2, and 4
b. choices 2, 7, and 8
c. choices 3 and 8 only
d. choices 2, 4, 7, and 8