A.Al3+, Cr3+.
B.Fe2+, Fe3+.
C.Ca2+, Mg2+.
D.Na+, K+.

A.saccarozơ.
B.glucozơ.
C.xenlulozơ.
D.tinh bột.

A.Nilon-6,6.
B.Amilopectin.
C.Xenlulozơ trinitrat.
D.Poli(vinyl clorua).

A.K.
B.Ca.
C.Ag.
D.Fe.

A.CnH2nO2.
B.CnH2n+2O2.
C.CnH2n-2O.
D.CnH2n-4O2.

A.HCOOC6H5; CH3COOC6H5.
B.HCOOC2H5; CH3COOC2H5.
C.HCOOC2H5; CH3COOC6H5.
D.HCOOCH3; HCOOC6H5.

A.was built
B.built
C.build
D.was building

A.The more difficult
B.The most difficult
C.Most difficult
D.Difficult

A.The more difficult
B.The most difficult
C.Most difficult
D.Difficult

A.does he
B.isn't he
C.was he
D.didn't he