What is the mass number of an element that has an atomic number of 20 and 21 neutrons?

Iknow the answer is a but i don't know why

Which supports the idea that birds and butterflies both have wings but they do not have a common ancestor with wings? a. the wings are analogous structures that evolved differently and do not have a similar internal structure. b. the wings are homologous structures that evolved differently and do not have a similar internal structure. c. wings of birds are vestigial structures, but the wing structures of bats are not vestigial. d. wings of bats are vestigial structures, but the wing structures of birds are not vestigial

What must happen before a body cell can begin mitotic cell division