Which colorless and odorless gas, produced by radioactive decay of uranium-238, is considered to be a form of pollution?

a. hydrogen
b. radon
c. oxygen
d. boron

Altocumulus, altostratus, cirrocumuls, cirrostratus, cirrus, cumulonimbus, cumulus, nimbostratus, stratocumulus, stratus 8.) clouds with tallest vertical development 9.) low altitude clouds in solid layers that produce rain 10.) high altitude thin flat layers of clouds 11.) rain clouds that form under stable atmospheric conditions 12.) middle altitude clouds with globular appearance