Photochemical smog requires sunlight, primary pollutants (such as nitrogen oxides and volatile organic compounds), and stagnant atmospheric conditions. Sunlight initiates photochemical reactions between the pollutants, leading to the formation of secondary pollutants like ozone. The stagnant air allows these pollutants to accumulate, exacerbating smog formation. Together, these ingredients create the conditions conducive to photochemical smog.
The plural form for the noun must is musts. The word must is also a verb, an auxiliary verb, and an adjective.
The changes to the rock must occur in solid form.
water vapour
Oxygen
For the oxidation of iron to occur, that is for ferric oxide to form, there must be oxygen present. There must also be water present.
The Sun
Cool air and dust Particles
High Relative Humidity
Pressure and heat.
Water vapor and heat must be present for clouds to form. The water is evaporated from the ground and collected into a cloud.
To form the present perfect tense, a past participle must be combined with the helping verbs, have, or has (present tense). In this case the verb is 'to be' and its past participle is 'been.' The answer is, "He has been ill."
If all elephants have trunks, then they must be present.