Final answer:
A restriction in the liquid line, such as dirt, would most directly be indicated by bubbling in the sight glass, though it could also cause high pressure upstream, low pressure downstream, and potentially a frosted evaporator coil. Option D is correct.
Step-by-step explanation:
If there's a restriction in the liquid line, such as dirt, it's likely to cause a high pressure on the upstream side of the blockage and a low pressure downstream. This is because the obstruction causes a drop in pressure as the refrigerant passes through the line.
If the liquid line is partially blocked, the reduced flow will not allow enough refrigerant to pass into the evaporator. Consequently, this could lead to a frosted evaporator coil if the refrigerant expands too much and drops in temperature below freezing.
However, the most direct indication of a restriction would be a bubbling sight glass, as it implies that not enough liquid refrigerant is reaching the evaporator, and gas bubbles are present in the sight glass as the refrigerant has not fully condensed
The correct answer is D) Bubbling sight glass. A bubbling sight glass would indicate a restriction, such as dirt, in the liquid line. When there is a restriction, the liquid refrigerant cannot flow freely, resulting in increased pressure and the formation of bubbles in the sight glass.
High pressure (option A) can be a consequence of a restriction, but it does not directly indicate the presence of dirt in the liquid line. Low pressure (option B) is not typically associated with a restriction. Frosted evaporator coil (option C) is a sign of a potential problem with the evaporator coil, not the liquid line restriction.