Down's syndrome due to trisomy of 21st chromosome is caused by

Question

$\begin{array}{1 1}(A)\;\text{Non-disjunction during egg formation}\\(B)\;\text{Non-disjunction during sperm formation}\\(C)\;\text{Addition of extra chromosome during mitosis of zygote}\\(D)\;\text{Either (A) or (B)}\end{array}$