frequency is equal to number of oscillations or vibrations upon time
therefore,
check picture
Answer:
A. Two tennis balls that are near each other
Explanation:
The formula for gravitational force (F) between two objects is

where m₁ and m₂ are the masses of the two objects, d is the distance between their centres, and G is the gravitational constant.
Thus, two objects that are far from each other will have a smaller gravitational force. We can eliminate Options C and D.
If the objects are at the same distance, those with the smaller mass will have a smaller force.
The mass of a tennis ball is 57 g.
The mass of a soccer ball is 430 g.
Two tennis balls that are near each other will have a smaller gravitational attraction.
Answer:
Hot water rises and cold water sinks is a model of thermal energy transfer by conduction.
A "screen" or even just a set of parallel bars are highly reflective to electromagnetic waves as long as the open spaces are small compared to the wavelengths.
"Grid" dishes work fine ... with less weight and less wind resistance ... for frequencies below about 3 GHz. (Wavelengths of at least 10 cm.)
(I even worked on a microwave system in South America where huge grid dishes were used on a 90-mile link.)