Verified answer

a. 355 m/s

Since speed of sound in air at 20°C is 343m/s and the speed of sound increases with temperature.

Oh, and that's the answer in the first source link.

Here's an extract from the first source:

In gases, an increase in temperature causes the molecules to move faster and this account for the increase in the speed of sound.

The formula to find the speed of sound in air is as follows:

v = 331m/s + 0.6m/s/C * T

v is the speed of sound and T is the temperature of the air [in Celsius]. One thing to keep in mind is that this formula finds the average speed of sound for any given temperature. The speed of sound is also affected by other factors such as humidity and air pressure.