Diamonds, an occasional component of rare igneous rocks called lamproites and kimberlites, have never been dated satisfactorily. However, some diamonds contain minute inclusions of silicate minerals, commonly olivine, pyroxene, and garnet. These minerals can be dated by radioactive decay techniques because of the very small quantities of radioactive trace elements they, in turn, contain. Usually, it is possible to conclude that the inclusions are older than their diamond hosts, but with little indication of the time interval involved. Sometimes, however, the crystal form of the silicate inclusions is observed to resemble more closely the internal structure of diamond than that of other silicate minerals. It is not known how rare this resemblance is, or whether it is most often seen in inclusions of silicates such as garnet, whose crystallography is generally somewhat similar to that of diamond; but when present, the resemblance is regarded as compelling evidence that the diamonds and inclusions are truly cogenetic.
17. The author implies that silicate inclusions were most often formed
(A) with small diamonds inside of them
(B) with trace elements derived from their host minerals
(C) by the radioactive decay of rare igneous rocks
(D) at an earlier period than were their host minerals（D）
(E) from the crystallization of rare igneous material
18. According to the passage, the age of silicate minerals included in diamonds can be determined due to a feature of the
(A) trace elements in the diamond hosts
(B) trace elements in the rock surrounding the diamonds
(C) trace elements in the silicate minerals
(D) silicate minerals’ crystal structure（C）
(E) host diamonds’ crystal structure
19. The author states that which of the following generally has a crystal structure similar to that of diamond?
(A) Lamproite
(B) Kimberlite
(C) Olivine
(D) Pyroxene（E）
(E) Garnet
20. The main purpose of the passage is to
(A) explain why it has not been possible to determine the age of diamonds
(B) explain how it might be possible to date some diamonds
(C) compare two alternative approaches to determining the age of diamonds
(D) compare a method of dating diamonds with a method used to date certain silicate minerals（B）
(E) compare the age of diamonds with that of certain silicate minerals contained within them