A.\(5\pi {a^2}.\)            
B.\(\sqrt 5 \,\pi {a^2}.\)    
C.\(2\sqrt 5 \pi {a^2}.\)    
D.\(10\pi {a^2}.\)

A.\(\dfrac{{1042}}{{225}}.\)
B.\(\dfrac{{208}}{{225}}.\)
C.\(\dfrac{{242}}{{225}}.\)
D.\(\dfrac{{149}}{{225}}.\)

A.\(7.\)
B.\(4.\)
C.\(5.\)
D.\(6.\)

A.Mg(OH)2.
B.MgCl2.
C.MgO.
D.Mg(NO3)2.

A.CO2.
B.N2.
C.O2.
D.H2.

A.Axit stearic.
B.Axit axetic.
C.Axit fomic.
D.Axit acrylic.

A.đồng.
B.kali.
C.nhôm.
D.magie.

A.Sn2+.
B.Cu2+.
C.Fe2+.
D.Ni2+.

A.HCl loãng.
B.HNO3 đặc, nguội.
C.Fe(NO3)3.
D.H2SO4 loãng.

A.CaCl2.
B.NaCl.
C.Na2SO4.
D.Ca(HCO3)2.