PROLIFERATION means to rapidly multiply. Which would concern you more – nuclear PROLIFERATION or the PROLIFERATION of bacteria on the International Space Station? I would vote for nuclear PROLIFERATION! But SAT test writers seem much more interested in the possibility of bacterial PROLIFERATION on the ISS. Although the station is a closed habitat, crew activities can produce localized humidity conditions that enable bacteria to PROLIFERATE. Who knew!
PROLIFERATION – to rapidly increase; multiply