$\begin{array}{1 1} (a)\; \text{$78 \%$ oxygen, $21 \%$ nitrogen, $1 \%$ other gases} \\ (b)\; \text{$21 \%$ oxygen, $78 \%$ nitrogen, $1 \% $other gases} \\ (c)\; \text{$79 \%$ oxygen, $20 \%$ nitrogen,$ 1 \%$ other gases} \\ (d)\; \text{$20 \%$ oxygen, $79 \%$ nitrogen, $1 \%$ other gases} \end{array}$
