During which prenatal period are the placenta and the umbilical cord formed

Energy derived from hot rocks and fluids beneath the earth’s surface is

What are some possible short and long term medical concerns for crew members on longer space exploration trips

Johnny rode his bike to a friend's house 4 blocks down the street in his neighborhood. he immediately rode back home once he realized his friend was not able to play. what was his displacement for the total bike ride? how did you determine this? what could we use as a reference point to determine he was in motion during his bike ride? why can you use it as a reference point