A.Mg.
B.Na.
C.Al.
D.Fe.

A.FeCl3.
B.FeCl2.
C.Fe(ClO4)3.
D.Fe(ClO4)2.

A.easy
B.expensive
C.acceptable
D.free

A.People
B.Hackers
C.Systems
D.Computers

A.spreading viruses
B.changing information
C.making a new password
D.manipulating codes

A.active hackers are more skilled.
B.passive hackers have more intense personalities.
C.active hackers do damage.
D.passive hackers are caught more easily.

A.need
B.start
C.encourage
D.design

A.need
B.start
C.encourage
D.design

A.They reduce the impact of transportation on climate.
B.Urbanization create severe problems in such cities.
C.There is more pollution in cities.
D.People in the cities face daunting challenges.

A.urban expansion
B.socio-economic disparities
C.disease
D.unsanitary conditions