Why are techniques like cluster sampling and multistage sampling just as externally valid as simple random sampling?

Due to the fact that they all contain elements of random selection, techniques like cluster sampling and multistage sampling are just as externally valid as simple random sampling due to the fact that they all contain elements of random selection.

Cluster sampling are more or less alike, each heterogeneous and resembling the overall population. The population is split into representative clusters, then a selection is done one or few from the clusters, then lastly, a census is performed with those selected clusters.

In multistage sampling, several methods are combined in sampling schemes and occasionally utilized the same method or different methods or more than once. Though they have different steps in being done, they both have random selection elements. 

25 meters

Step-by-step explanation:

in this case they give us the information about the lake and that length corresponds to the perimeter.

the perimeter of a circle has the following formula:

p = 2 * pi * r

then r equals:

r = p / pi * 2

replacing we have:

r = 157 / (2 * 3.14)

r = 25

which means that the radius of the lake is 25 meters