Space travel is definitely bad for astrona

题目：

Space travel is definitely bad for astronauts’ bones, reducing their bone density after only a month of weightlessness, according to French research published on Friday.
Laurence Vico and his fellow workers at St Etienne University called for more research into the effects of microgravity, after their study of 15 astronauts from the Russian MIR station showed bone loss continued throughout space flights.
"Bone loss was especially striking in four astronauts, " the scientists reported in the Lancet Medical Journal.
They measured the Bone Mineral Density (BMD) of bones in the forearm and lower leg of the astronauts who had spent one to six months in space.
The BMD loss was significant in the tibia of the lower leg, a weight-bearing bone, but barely changed in the radius of the forearm. "Our results indicate the need to investigate not only different bones, but also different areas of the same bone since not all sites of the skeleton are similarly affected by space conditions, " they added.
Without gravity the body isn’t bearing any weight so there is no need for calcium which makes bones p, and it becomes empty into the bloodstream.
The research team suggested in future scientists should try to determine if the loss of bone density was only on weight-bearing bones on longer flights, also the possible recovery after returning to Earth.

French scientists did their research on Russian astronauts, because ______.

A. they only cared for the Russian astronauts
B. they were not interested in their own astronauts
C. the Russian government invited them to do their research
D. the Russian astronauts worked in space for a long time