The number of elements in set A = 10
The number of elements in set B = 9
for the mapping to be onto there will be a element in set B which has 2 pre-images from set A
Two elements can be selected in $$^{10}C_2 $$ ways and the elements in set B can be arranged in 9! waysÂ
$$\therefore$$ No of onto functions =Â $$^{10}C_2 \times 9!$$
A is the correct answer
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