Sampling and Experiments · Superchallenge problem

A street has 100 houses numbered 1 to 100; the houses numbered $10, 20, \ldots, 100$ are corner houses. Plan A is a systematic sample of 20 houses: pick a random start from 1 to 5 and take every 5th house. Plan B is a simple random sample of 20 houses. Let $C$ be the number of corner houses in the sample. How many times larger is $\mathrm{Var}(C)$ under Plan A than under Plan B?