4. 阅读理解 The BeiDou Navigation Satellite System（BDS）is a Chinese satellite navigation system. It consists of two separate satellite constellations（星座）. The first BeiDou system, officially called the BeiDou Satellite Navigation Experimental System and also known as BeiDou-1, consists of three satellites, which since 2000 has offered limited coverage and navigation services, mainly for users in China and neighboring regions. Beidou-1 was decommissioned at the end of 2012.
The second generation of the system, known as COMPASS or BeiDou-2, became operational in China in December 2011 with a partial constellation of 10 satellites in orbit. Since December 2012, it has been offering services to customers in the Asia-Pacific region.
In 2015, China started the build-up of the third generation BeiDou system（BeiDou-3）in the global coverage constellation. The first BDS-3 satellite was launched on 30 March 2015. Up to January 2018, nine BeiDou-3 satellites have been launched. BeiDou-3 will eventually consist of 35 satellites and is expected to provide global services upon completion in 2020. So far, China has sent 27 BeiDou satellites into space. When fully completed, BeiDou, the world's fourth navigation satellite system, following GPS in the United States, GLONASS in Russia and Galileo in the European Union, will provide an alternative global navigation satellite system to the United States owned Global Positioning System（GPS）, and is expected to be more accurate than the GPS. It was claimed in 2016 that BeiDou-3 will reach millimeter-level accuracy（with post-processing）, which is ten times more accurate than the finest level of GPS.
According to China Daily, in 2015, fifteen years after the satellite system was launched, it was generating a turnover of $31.5 billion annually for major companies such as China Aerospace Science and Industry Corp, AutoNavi Holdings Ltd, and China North Industries Group Corp.
(1)、What does the underlined word “decommissioned” in the 1st paragraph mean?
A、out of service
B、out of control
(2)、How many BeiDou-3 satellites had been sent into space by January 2018?
(3)、Which of the following statements about BDS is true according to the passage?
A、Beidou-2 began to operate in December, 2012.
B、Beidou-1 cannot cover all the Asia-Pacific regions.
C、Beidou-3 had reached the finest accuracy of GPS by 2016.
D、BDS ranks fourth among the existing navigation satellite systems in the world.
(4)、What is the passage mainly about?
A、The service of BDS.
B、The accuracy of BDS.
C、The development of BDS.
D、The comparisons among the satellite systems.