It should be D, cells are parts, tissue is part and organ is part of a system.
        
                    
             
        
        
        
No Ag cannot react with NaOH because Ag is less reactive than Na in the reactivity series and can't displace it
 
        
             
        
        
        
An object that is traveling around another body in space is in ORBIT around that body
        
             
        
        
        
1.35atm(760mmHg/1atm)=1026mmHg
        
             
        
        
        
Element   Atomic Number	Valency
Valency of Hydrogen	1	1
Valency of Helium	2	0
Valency of Lithium	3	1
Valency of Beryllium	4	2
Valency of Boron	5	3
Valency of Carbon	6	4
Valency of Nitrogen	7	3
Valency of Oxygen	8	2
Valency of Fluorine	9	1
Valency of Neon	10	0
Valency of Sodium (Na)	11	1
Valency of Magnesium (Mg)	12	2
Valency of Aluminium	13	3
Valency of Silicon	14	4
Valency of Phosphorus	15	3
Valency of Sulphur	16	2
Valency of Chlorine	17	1
Valency of Argon	18	0
Valency of Potassium (K)	19	1
Valency of Calcium	20	2
Valency of Scandium	21	3
Valency of Titanium	22	4
Valency of Vanadium	23	5,4
Valency of Chromium	24	2
Valency of Manganese	25	7, 4, 2
Valency of Iron (Fe)	26	2, 3
Valency of Cobalt	27	3, 2
Valency of Nickel	28	2
Valency of Copper (Cu)	29	2, 1
Valency of Zinc	30	2