${ }^{20} \mathrm{C}_{\mathrm{m}}+{ }^{20} \mathrm{C}_{\mathrm{m}-1}=$ কোনটি?

${ }^{21} \mathrm{C}_{\mathrm{m}}$

${ }^{20} \mathrm{C}_{\mathrm{m}}$

${ }^{21} \mathrm{C}_{\mathrm{m}-1}$

${ }^{19} \mathrm{C}_{\mathrm{m}}$