I think it is but 1. Element symbol
Answer:

Explanation:
Given:
temperature at the hotter end, 
temperature at the cooler end, 
length of rod through which the heat travels, 
cross-sectional area of rod, 
mass of ice melted at zero degree Celsius, 
time taken for the melting of ice, 
thermal conductivity k=?
By Fourier's Law of conduction we have:
......................................(1)
where:
=rate of heat transfer
dT= temperature difference across the length dx
Now, we need the total heat transfer according to the condition:
we know the latent heat of fusion of ice, 



Now the heat rate:



Now using eq,(1)


The
sun is a ball of hot gases containing different kinds of elements at different
cores. It has a very high temperature that radiates all throughout the Milky
Way galaxy. The sun has three main parts; photosphere, chromospheres
and corona. The outer core of a star located at the chromospheres contains
mostly of hydrogen. Inside the hydrogen is helium then carbon, oxygen, neon,
magnesium silicon and the inert gas. The photosphere is scattered by the loose electrons in the corona’s plasma.
Answer:
A vacuum
Explanation:
Sound waves are examples of mechanical waves. Mechanical waves are waves which are transmitted through the vibrations of the particles in a medium.
For example, sound waves in air consist of oscillations of the air particles, which vibrate back and forth (longitudinal wave) along the direction of propagation of the wave itself.
Given this definition of mechanical wave, we see that such a wave cannot propagate if there is no medium, because there are no particles that would oscillate. Therefore, among the choices given, the following one:
a vacuum
represent the only situation in which a sound wave cannot propagate through: in fact, there are no particles in a vacuum, so the oscillations cannot occur. In all other cases, instead, sound waves can propagate.
C. is the only double reaction here given that a double replacement reaction involves two compounds that exchange previous components, and C is the only solution with two compounds present