Rigid skeletons are formed from materials including chitin (in arthropods), calcium compounds such as calcium carbonate (in stony corals and mollusks) and silicate (for diatoms and radiolarians).
While there is no graph provided and I cannot find the graph online, I will base my answer through theory.
In theory, the most common blood type is O+, followed by A+, then B+, then AB+. Rh negative blood is deemed to be rare. While the most common blood type is O+, the same blood type is also called the "universal donor" as a type O+ blood has no antigens attached in red blood cells therefore the probability of adverse reactions (i.e. hemolytic reactions) when transfused to other blood types are close to nil. In an emergency situation, wherein there is not enough time to do proper crossmatching, blood type O+ can be used therefore supplies of blood type O+ are easily depleted.
Answer:
there could have been low production
