Which are true of the $($counting$)$ semaphore operation called "wait$()"?$

Which are true of the $($counting$)$ semaphore operation called "wait$()"?$

it decrements the value of the semaphore

it increments the value of the semaphore

if the value becomes negative, the calling process is blocked

if the value is not positive, another process is unblocked

A lower$-$level mutual exclusion mechanism is needed, to ensure that the fetching and storing of the semaphore value are done atomically.