One Difference Between a Confounding Variable and a Proxy Variable

One difference between a confounding variable and a proxy variable is that:

A) confirming causal relationships is made more difficult by confounding variables and easier by proxy variables
B) confirming causal relationships is made more difficult by proxy variables and easier by confounding variables
C) confounding variables cannot be studied at all, while proxy variables can
D) proxy variables cannot be studied at all, while confounding variables can
E) confounding variables occur in nature, whereas proxy variables are always put in place by humans