Questions

FCH₂COOH is the strongest acid out of all.

Since the electron withdrawing inductive effect (-I effect) of the halogens decreases in the order of F>Cl> Br> I , therefore the acidic strength of the α- halo acids decreases in the same order.

FCH₂COOH  >  ClCH₂COOH   >   BrCH₂COOH   >   ICH₂COOH.

The man has 12 sons and the 12 sons has each one sister.

=> 12 sons + 1 sister = 13 children.

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The common greenhouse gas containing only hydrogen and oxygen is H2O, this is water vapor.

The common greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and ozone.

The composition (chemical formula) of those gases are:

Water vapor: H2O
Carbon dioxide: CO2
Methane: CH4
Nitrous oxide: NO
Ozone: O3.

As you see the only one of those that contains only hydrogen and oxygen is H2O, water vapor.