The answer is D. Seaweed.
The air spaces occur in Spongy mesophyll tissue.There are large spaces in a leaf because it is for storing water and carbon dioxide which will be used for photosynthesis. The large air spaces are usually found in the spongy layer of the mesophyll. The arrangement of this spongy mesophyll facilitates the movement of gases through the mesophyll. Directly beneath each stomate at the leaf surface there is usually a small air space, called a stomatal chamber, where no mesophyll cells are present. This also aids in gas exchange.
(C) they used trans-Saharan trade route to acquire expensive ivory.
Pyroclastic materials are classified according to their size, measured in milli meters: dust (less than 0.6 mm [0.02 inch]), ash (fragments between 0.6 and 2 mm [0.02 to 0.08 inch]), cinders (fragments between 2 and 64 mm [0.08 and 2.5 inches], also known as lapilli), blocks (angular fragments greater than 64 mm), and bombs (rounded fragments greater than 64 mm).
The fluid nature of a pyroclastic flow is maintained by the turbulence of its internal gases. Both the incandescent pyroclastic particles and the rolling clouds of dust that rise above them actively liberate more gas. The expansion of these gases accounts for the nearly frictionless character of the flow as well as its great mobility and destructive power.
Pyroclastic flow, in a volcanic eruption, a fluidized mixture of hot rock fragments, hot gases, and entrapped air that moves at high speed in thick, gray-to-black, turbulent clouds that hug the ground. The temperature of the volcanic gases can reach about 600 to 700 °C (1,100 to 1,300 °F). The velocity of a flow often exceeds 100 km (60 miles) per hour and may attain speeds as great as 160 km (100 miles) per hour.
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