3. Which of these organs is directly involved in synthesis of red blood cells in an adult?
A. Liver
B. Spleen
C. Bone marrow
D. Lymph nodes

*Answer*: C.Bone marrow
*Explanation*: In the human adult, the bone marrow produces all of the red blood cells, 60–70 percent of the white cells (i.e., the granulocytes), and all of the platelets.

Why the answer not A,B&D:

The lymphatic tissues, particularly the thymus, the spleen, and the lymph nodes, produce the lymphocytes (comprising 20–30 percent of the white cells). The reticuloendothelial tissues of the spleen, liver, lymph nodes, and other organs produce the monocytes (4–8 percent of the white cells). The platelets, which are small cellular fragments rather than complete cells, are formed from bits of the cytoplasm of the giant cells (megakaryocytes) of the bone marrow.

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Science27 of the most iconic, jaw-dropping photos of the earth and the moon from spacedave mosher  oct 1, 2018, 11: 13 am etnasa/flickrapollo 11 astronauts took this photo of earth on july 20, 1969.from the ground, earth looks like a boundless fertile plain that beckons to be explored and exploited.but astronauts would beg -  and even plead  - to differ."you realize that people often say, 'i hope to go to heaven when i die,'" jim lovell, an astronaut who flew on the apollo 8 and apollo 13 missions to the moon, previously  told business insider. "in reality, if you think about it, you go to heaven when you're born."lovell has seen first-hand that we live on a tiny rock hopelessly lost in the void. he's also quick to tell you it's the only one we've got - a  fragile spaceship  for more than 7 billion people: "you arrive on a planet that has the proper mass, has the gravity to contain water and an atmosphere, which are the very essentials for life. and you arrive on this planet that's orbiting a star just at the right distance - not too far to be too cold, or too close to be too hot - and just at the right distance to absorb that star's energy and then, with that energy, cause life to evolve here in the first place. in reality, you know, god has really given us a stage, just looking at where we were around the moon, a stage on which we perform. and how that play turns out is up to us, i guess."humanity has recorded photos of earth from hundreds, thousands, millions, and even billions of miles away, some of them taken by lovell himself.these images not only scientists study our dynamic world, but also understand how a habitable planet looks from afar, which aids the search for  more worlds. most importantly, however, the images underscore our peculiar existence on  a mote of cosmic dust.take a moment to ponder 27 of the most arresting images of earth that humankind has ever captured from space.a