This is the order of activity in decreasing way for the mentioned metals
Mn>Zn>Cr>Fe>Cd>Co>Ni>Pb
Fe is in between Cr and Cd, it is more active than Cd and less active than Cr, so it will displace Cd but not Cr.
Answer:Fe (it will displace Cd, but not Cr)
It is c because I did it already
Answer: The products usually weigh more than the reactants.
Explanation:
When it is filled with Helium it will expand and become more dense in the hot car. But when it becomes dense the gas molecules fly around in different directions, they collide with other objects and create pressure which can cause the balloon to pop.