C. Three isotopes of the same element because the number of electrons and protons remain the same but the number of neutrons in the nucleus are different. This means that the isotopes have different masses to each other but the same chemical properties.
Answer:B
Explanation:
Given
mass of Sled is m
another package of mass m is thrown on it
Suppose u be the initial velocity of sled
conserving momentum
where v is the final velocity
Initial kinetic energy 
Final Kinetic Energy
Final Kinetic Energy
Final kinetic Energy is half of initial
Hookes law state that provided that the elastic limit is not exceeded, the extension is directly proportional to the force
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<span>a=Δω/Δt
</span><span>a=2π*Δf/Δt
</span><span>a=2π*(f2-f1)/Δt
</span>
<span>f1=f2-a*Δt/2π
</span><span>f2=800/60 rev/sec
</span><span>a=-42 rad/sec^2
</span><span>Δt=1.75sec
</span><span>so
f1=25 rev/sec
f1=1500 rev/min</span>
