A. Human error
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So its acquired immunity and the cells responsible are the B lymphocytes that produces antibodies against the virus. Memory is keep since first encounter of the virus by the immune system by way of the antibodies that was initially produced. The antibodies are there already, which makes it fast to act upon when expose again.
A convection oven cooks food with the help of A built-in fan that circulates hot air circulates air around and over the food.
About 74% of earth's surface is covered by water.
