13.According to the passage,scientists do not fully understand why double radio sources(A)have not eventually disappeared

(B)cannot be observed with a telescope

(C)are beginning to slow down

(D)are not as big as some planets and stars

14.The word "their"in line 22refers to

(A)speeds

(B)directions

(C)electrons

(D)clouds

15.According to the passage,what happens when electrons and gas collide in space?

(A)The gas becomes more condensed

(B)The gas becomes less radiated

(C)The electrons disperse

(D)The electrons become negatively charged

16.The author suggests that astronomers consider the study of cosmic jets to be

(A)an obsolete scientific field

(B)an unprofitable venture

(C)an intriguing challenge

(D)a subjective debate

17.In what lines does the passage compare the size of double radio sources with that of other galaxies?

(A)Lines 4-6

(B)Lines 12-14

(C)Lines 19-20

(D)Lines 23-24

18.Where in the passage does the author mention a technology that aided in the understanding of double radio sources?

(A)Line 2

(B)Line 7

(C)Line 17

(D)Line 21