$81 = $ ?

$3^{4} \times 13$ $3^{4}$ $3^{4} \times 17$ $3^{2}$ $3^{5}$

$840 = $ ?

$2^{3} \times 3 \times 5 \times 7 \times 19$ $2^{3} \times 3 \times 5 \times 7$ $2^{4} \times 3 \times 5 \times 7$ $2^{3} \times 3 \times 5 \times 7^{2}$ $2^{2} \times 3 \times 5 \times 7$

$189 = $ ?

$3^{3} \times 7$ $3^{3} \times 7^{2}$ $3^{3} \times 7 \times 13$ $3 \times 7$ $3^{2} \times 7$

$42 = $ ?

$2 \times 3 \times 7 \times 13$ $2 \times 3 \times 7 \times 11$ $2 \times 3 \times 7$ $2 \times 3 \times 5 \times 7$ $2 \times 3 \times 7^{2}$

$20 = $ ?

$2^{3} \times 5$ $2^{2} \times 5$ $2^{2} \times 5 \times 19$ $2 \times 5$ $2^{2} \times 5 \times 17$