It is now established that the Milky Way is far more extended and of much greater mass than was hitherto thought. However, all that is visible of the constituents of the Milky Way’s corona (outer edge), where much of the galaxy’s mass must be located, is a tiny fraction of the corona’s mass. Thus, most of the Milky Way’s outlying matter must be dark.
Why? Three facts are salient. First, dwarf galaxies and globular clusters, into which most of the stars of the Milky Way’s corona are probably bound, consist mainly of old stars. Second, old stars are not highly luminous. Third, no one has detected in the corona the clouds of gaseous matter such as hydrogen and carbon monoxide that are characteristic of the bright parts of a galaxy. At present, therefore, the best explanation—though still quite tentative—for the darkness of the corona is that the corona is composed mainly of old, burned-out stars.
17. The passage as a whole is primarily concerned with
(A) analyzing a current debate
(B) criticizing a well-established theory
(C) showing how new facts support a previously dismissed hypothesis
(D) stating a conclusion and adducing evidence that may justify it（D）
(E) contrasting two types of phenomena and showing how they are related
18. According to the passage, a bright part of a galaxy typically includes
(A) dwarf galaxies and clusters of stars
(B) a balanced mixture of old and new stars
(C) a large portion of the galaxy’s mass
(D) part of the corona of the galaxy（E）
(E) gases such as hydrogen and carbon monoxide
19. It can be inferred from the passage that, compared with what they now think, until fairly recently astronomers believed that the Milky Way
(A) was much darker
(B) was much smaller
(C) was moving much more slowly
(D) had a much larger corona（B）
(E) had much less gaseous matter
20. The passage presents which of the following as incontrovertible?
I. The low luminosity of old stars
II. The absence of clouds of gaseous matter from the corona of the Milky Way
III. The predominance of globular clusters and dwarf galaxies in the corona of the Milky Way
(A) I only
(B) III only
(C) I and II only
(D) II and III only（A）
(E) I, II, and III