Achieving a balanced diet is best done by
a. eating nothing but fruits.
b. getting the right number of calories each day.
c. eating at least one item from each food group daily.
d. getting 100% of your calcium requirements each day.

B. eating the right amount of calories each day
for a, you need proteins,veggies and calories to survive, as each does a specific job such as glucose and protein being created, etc.
For c, you can have at least 1 item from each food group each day, but you can still have too much of something and thats harmful.
For d,getting 100% calcium only works with calcium, and  you can do anything with the other foods.
For b(which is correct), people usually eat an excess of calories, and that mainly controls your weight. Now other food groups can also affect you if you have too much, but people tend to have too much calories, so  getting the right amount of calories is the best choice

Helium balloons are made of less porous material. Air is a mixture of oxygen and nitrogen molecules which are much larger and heavier than helium atoms that is why they deflate quicker than air filled balloons.