<span>You are trying to find V2, the formula is V2</span>= <span><span>P1 / </span><span>P2 </span></span>⋅ V<span>1
</span><span>input your numbers, V2</span>= <span><span>90.0kPa / </span><span>20.0kPa </span></span>⋅ 4.0 L to get 18 L
B. Sand.
Sand would definitely increase friction because of the roughness of it surface. When its surface comes with another rough surface it would tend to increase friction. All other options given tend to reduce friction one way or the other.
An Insulator.
Insulators prevent flow of electric current. Example is wood, clay.