The answer is A.
Twins form in one of two ways:
Identical twins occur when a single fertilized egg splits into two.
Fraternal twins occur when two, separate eggs are fertilized by two, separate sperm.
The answer is fusion of gametes via fertilization.
Matthias Jacob Schleiden was a German botanist who, with Theodor Schwann, cofounded the cell theory. In 1838 Schleiden defined the cell as the basic unit of plant structure, and a year later Schwann defined the cell as the basic unit of animal structure.
