Based on the information in figure 2 which substances would be gas is at 0°c

As we can see from the table that boiling point of Nitrogen and Hydrogen is -195.8 and -252.9 degree Celsius.

So at 0 degree Celsius both of the gas will reach to the boiling point and will convert into the gaseous state

while other two substance will remain in liquid state or solid state at 0 degree C because they both have greater boiling temperature or boiling temperature greater than zero.

So the answer would be

Nitrogen

Hydrogen

wavelength divides in half and speed stays the same.