Which of the following underlined words or phrases is incorrect?

The first electric lamp had two carbon rods from which vapour serves to conduct the current across the gap.

Choose the incorrect part:
Cans of paint must be shaking to mix the pigments with the medium in which in which they are suspended.

This sentence has four underlined words or phrases (A, B, C, and D). Identify the word or phrase that must be changed in order for the sentence to be correct:

Policy-makers award a green label to products that meet certain environmental criterion.

identify the incorrect one from the underlined sections A ray of light passing through the center of a thin lens keep its original direction.

Although both $\frac{of ~them}{\left(A\right)}$ $\frac{are\ trying}{\left(B\right)}$ $\frac{to\ get}{\left(C\right)}$ the scholarship she has the $\frac{highest}{\left(D\right)}$ grades