Answer:
D) 7 before and 7 after
Step-by-step explanation:
Na₂O + 2HCl ⟶ 2NaCl + H₂O
3 atoms + 4 atoms ⟶ 2×2 atoms + 3 atoms
7 atoms ⟶ 4 atoms + 3 atoms
7 atoms ⟶ 7 atoms
There are seven atoms present before and after the reaction.
The probability is 12.5% or 1/8 of getting heads in the first two trials and tails in the last trial for flipping the coin.