You go outside at 6 p. m. and see the moon on the western horizon. what phase is it?

If the moon is near the western horizon at 6 PM, then it's quite
near the sun in the sky.  But from the scanty information given
in the question, we don't know whether it's a little bit ahead of
the sun or behind the sun.

In either case, the moon is a crescent.

-- If the moon is slightly ahead of the sun, then it's a waning crescent.
It will set while the sun is still in the western sky, and it'll be very hard
to see it.

-- If the moon is slightly behind the sun, then it's a waxing crescent.
The sun will set a short time before the moon, leaving the beautiful
crescent moon low in the dimly lit western sky.

Answer volcanic outgassing;