题目内容：

On a clear night, you can see a sky paced with stars. But you can’t see planets orbiting any of these stars, even with the strongest Earth—based telescopes. Astronomers reported that they discovered seven Earth—sized planets orbiting a nearby star called TRAPPIST – 1 in February. They had to rely on indirect evidence to tell them that the planets are there. By doing some math, they figured out the number of planets and the distance of their orbits.

TRAPPIST – 1 is a red dwarf （红矮星）, much smaller than our sun, but larger than Jupiter (木星). If the sun were the size of a basketball, TRAPPIST – 1would be the size of a gold ball. This star is one of our closest neighbors, just like the moon to the earth in space.

When astronomers using Earth-based telescopes to study TRAPPIST – 1,they noticed that every once in a while, it became slightly dim（昏暗）. They decided that a planet must be passing in front of the star, blocking part of the light. This dimming would come and go as the planet orbited the star, passing alternately (交替地) behind and in front of the star regularly.

By making careful observations, astronomers expected to get a rough idea of the planet’s size from the pattern of dimming light. But the light level changed often and not at evenly spaced (均匀间隔的) times. The astronomers had to conclude they were looking at more than one planet! To get additional data, NASA’s spritzer Space Telescope measured TRAPPIST –1’s brightness nearly continuously for 20 days, catching 34 crossings of planets in front of the star. Sorting through the mess of data, the astronomers found that the innermost planet completes its orbit in 1.5 Earth days, while the outermost planet takes 20 days. Once you know how long it takes each planet to complete an orbit, you can work out how far it is from the star compared with the other planets.

The TRAPPIST – 1 planets are not alone. So far, astronomers have found more than 3,400 planets that orbit other stars.

1.How did astronomers know the number of planets orbiting TRAPPIST –1?

A. They used their eyes.    B. They used telescopes.

C. They performed some calculation.    D. They made some guess.

2.How does the author introduce TRAPPIST – 1 in paragraph 2?

A. By giving examples.    B. By making comparisons.

C. By providing research results.    D. By listing some numbers.

3.What probably causes the star TRAPPIST – 1 to be slightly dim?

A. The telescope’s failure.    B. The cloud floating around.

C. The star’s moving.    D. The passing planets.

4.What’s the main idea of Paragraph 4?

A. The ways to get the information of the planets.

B. The introduction of the planets.

C. The distances of the planets to the star.

D. The astronomers’ effort of exploring TRAPPIST – 1.