Please help me understand this question & answer correctly! thank you so much. this is more specifically materials engineering if that helps (: i will upvote

Identify the types of cast iron based on the following microstructures. A B C D O A: Gray cast iron, B: White cast iron, C: Ductile (Nodular) cast iron, D: Malleable cast iron O A: White cast iron, B: Malleable cast iron, C: Gray cast iron, D: Ductile (Nodular) cast iron O A: Gray cast iron, B: Ductile (Nodular) cast iron, C: White cast iron, D: Malleable cast iron O A: Ductile (Nodular) cast iron, B: Gray cast iron, C: Malleable cast iron, D: White cast iron