Answer:
 I want to say option C: Testing which surfa e is easier to slide a wooden block across. 
Explanation:
I'm not 100 % sure though with my answer. I'm truly so very sorry if my answer is wrong. I tried my best.
 
        
             
        
        
        
The mass decay rate is of the form

where 
m₀ = 3000 g,the initial mass
k = the decay constant
t = time, years.
Because the half-life is 30 years, therefore

After 60 years, the mass remaining is

Answer: 750 g
 
        
             
        
        
        
The hydrogen and oxygen<span> atoms from H</span>₂O are <span>bonded together through covalent </span>bonding.