更多"[单选题]Local authorities have to lear"的相关试题:
[单选题]The older New England villages have changed relatively little ___________ a gas station or two in recent decades.
A.except for
B.except
C.in addition
D.besides
[单选题]Food seems to have been （ ） in the local markets, even if the quality left much to be desired.
A.available
B.avoidable
C.adorable
D.advisable
[单选题]Two events have highlighted the tensions in recent years.
A.extended
B.emphasized
C.distorted
D.illustrated
[不定项选择题]共用题干
Learn About Noble Gases（惰性气体）
1 Have you ever ridden on a balloon?Many tourist spots offer balloon rides in order for people to see
the beauty of a place from above.A balloon contains a noble gas called helium（氦）.Formerly,balloons con-
tamned hydrogen but hydrogen is very flammable and dangerous when uncontrolled.Therefore,people shifted
to helium,which is safer. Helium is safe because it has the properties of the noble gases.
2 People once believed that noble gases couldn't chemically react at all.For this reason,they were
called inert gases（惰性气体）.They were also listed under Group 0 in the old periodic table because scien-
tists believed that the gases have zero valence（价）electrons in their outer shell.This was later proven to be
untrue when some noble gas compounds were discovered.
3 The gases are elements,which share similar properties.These properties include being monoatomic,
colorless,odorless,being able to conduct electricity,and having low chemical reactivity.Noble gases
include helium,neon,argon,krypton,xenon and radon.These are all found in Group 18,in the right most
column of the periodic table.If you look at the periodic table,you will notice that these elements are the only
ones which do not have a charge. Helium has the lowest molecular（分子的）weight while radon is the
heaviest.
4 Remember that chemical reactions occur because atoms have valence electrons,which are electrons
in their outer shell.When the outer shell is"unfilled"or the required number of electrons is not yet corn-
plete,the atom is more reactive.Noble gases have a full outer shell,meaning that they have complete elec-
trons in their outer shell.This complete number varies.For instance,the outer shell of helium has 2 valence
electrons while the outer shell of xenon has 8 electrons.Nowadays,there remains to be a few noble gases
because of the low chemical reactivity of these said gases.
5 because of their properties,noble gases have many important applications.They are widely used in
medicine and industries. For instance , liquid helium is used for superconducting magnets（磁体）.These
magnets are very important in physics and medicine.When a doctor suspects that a person's brain has been
damaged,he might request for Magnetic Resonance Imaging(MRI).MRI allows the doctor to"see"the
brain,without operating on the patient.
MRI may make operating on the patient____________.
A.complete
B.reactive
C.unnecessary
D.flammable
E.important
F.lightest
