In each of the following examples a strong correlation, either negative or positive, holds between two sets of phenomena, $A$ and $B$. Try to decide whether $A$ is the cause of $B, B$ is the cause of $A$, both are caused by some third factor, $C$, or the correlation is simply accidental. Explain your choice.
1. For a particular United States president, there is a negative correlation between the number of hairs on his head $(A)$ and the population of China $(B)$.
2. My son's height (A) increases along with the height of the tree outside my front door (B).
3. It has been claimed that there is a strong positive correlation between those students who take sex education courses (A) and those who contract venereal disease (B).
4. At one time there was a strong negative correlation between the number of mules in a state (A) and the salaries paid to professors at the state university (B). In other words, the more mules, the lower professional salaries. ${ }^6$
5. There is a high positive correlation between the number of fire engines in a particular borough in New York City (A) and the number of fires that occur there (B). ${ }^7$
6. "Washington (UPI)-Rural Americans with locked doors, watchdogs or guns may face as much risk of burglary as neighbors who leave doors unlocked, a federally financed study says. The study, financed in part by a three-year $$\$ 170,000$$ grant from the Law Enforcement Assistance Administration, was based on a survey of nearly 900 families in rural Ohio. Sixty percent of the rural residents surveyed regularly locked doors $[A]$, but were burglarized more often than residents who left doors unlocked $[B] . .^{\prime \prime s}$
7. The speed of a car (A) is exactly the same as the speed of its shadow (B).
8. The length of a runner's ring finger minus the length of the runner's index finger $(\mathrm{A})$ is correlated with the runner's speed in the one-hundredyard dash (B).