Consider the following simple regression model: y$=$$0+$$1$x$+$u$.$

Suppose z is an instrument for x$.$

Which of the following conditions denotes instrument relevance?

Group of answer choices

The instrumental variables estimator is always biased if Cov$($x$,$u$)0.$

The condition Cov$($z$,$u$)=0$ can be tested statistically.

The condition Cov$($z$,$x$)0$ cannot be tested statistically.

The ordinary least squares estimator is unbiased if Cov$($x$,$u$)0.$