Do you have any knowledge of the space station？ A space station is a place 【21】______ peop

le can and work on space 【22】______ long periods. It orbits the earth, usually about 200 to 300 miles high. A space station may 【23】______ as an observatory, laboratory, factory, workshop, warehouse, and fuel depot. Space stations are much larger than manned spacecraft, 【24】______ they provide more comforts. Manned spacecraft may 【25】______ people between the earth and the space station. Unmanned spacecraft may supply the station 【26】______ food, water, equipment, mail and so on.

Small space stations can be 【27】______ on the earth and launched orbit by 【28】______ rockets. Larger stations are 【29】______ in space. Rockets or space shuttles 【30】______ modules of the station into space, where astronauts assemble them. Old modules can be replaced, and new modules can be 【31】______ to expand the station.

A space station has 【32】______ one docking port 【33】______ which a visiting spacecraft can attach itself. Most docking ports 【34】______ of a rimmed doorway called a hatch that can connect 【35】______ the hatch on the visiting spacecraft to form. an airtight seal. When the two hatches open, they form. a pressurized tunnel between the station and the visiting spacecraft.

The main tasks of a space station crew involve scientific research. For example, they might analyze the 【36】______ of micro gravity on various materials, 【37】______ the earth's surface, or study the stars and planets.

Astronauts 【38】______ a space station also 【39】______ much of their time to the assembly of equipment and the expansion of the station's facilities. This includes 【40】______ beams, connecting electrical and gas lines, and welding permanent joints between sections of the station. The crew must also fix or replace broken equipment.

【21】

A．which

B．that

C．where

D．in which