Read the following passage. Fill in each blank with a proper sentence given in the box. Each sentence can be used only once. Note that there are two more sentences than you need.

Spacewalk!

    Most people don't know that the anniversary of an important event in space exploration occurred last month. On March 18, 1965, spaceman Aleksi Leonov became the first human to complete an Extra Vehicular Activity (EVA) or spacewalk. It marked the first time that a human was able to leave a spacecraft and operate in the emptiness of space. It is a dangerous procedure, but one that is vital for the success of manned space missions.

     In space, a spacesuit must protect people from extreme cold and heat, provide air to breathe, and remove extra carbon dioxide.  Too much of it, and the spacesuit becomes firm and difficult to move in. Too little of it, and astronauts can become dangerously sick after returning to their spacecraft.

    Astronauts now perform complex jobs in the emptiness of space in modern spacesuits. They have logged many hours repairing and upgrading equipment on satellites during EVAs.  On July 20^th, 1969, American astronaut Neil Armstrong set foot on the moon for the first time. There is no atmosphere on the moon, but there is gravity—about 17 percent of Earth's, which means Neil needed a special suit for walking on the moon's surface. Suits for the moon are equipped for exploration far from any vehicles, including tough boots that can resist cuts from the rough surface while walking. But sharp rocks weren't the only danger to astronauts.

    The moon surface is covered with a fine and flour-like dust which consists of smallparticles(颗粒) left over from the numerousmeteorite(陨石) strikes on the moon.  When astronauts would leave the moon's gravity, the dust on their suits began floating all over. It got into delicate equipment inside the spacecraft and the astronaut's eyes and lungs. As different space agencies plan for returning people to the moon, new EVA suit designs will have to take something else into account. Keeping astronauts safe also means keeping their suits clean.

A. However, EVAs don't just happen in empty space.

B. It sticks to everything, eventually causing joints and seals to fail.

C. Spacesuits also have to maintain ideal air pressure during a spacewalk.

D. Unfortunately, most people have no idea about space exploration or spacesuits.

E. Without the ability to work in space, we are unable to keep our space programs functional.

F. There are a few things that must be taken into account to make an EVA both safe and effective