When troubleshooting a refrigeration system, the engineer observes low suction pressure, low head pressure, bubbles in the sight glass, and frost forming only partway along the evaporator. Which condition BEST explains all of these symptoms?

Why C is correct

An undercharge (refrigerant shortage or leak) produces the combination of low suction pressure, low head pressure, bubbles in the sight glass, poor cooling, and frost only partway along the evaporator. A plugged filter-drier also causes low suction pressure but typically shows a temperature drop and frost at the restriction point. A stuck-open expansion valve causes slugging and a frosted suction line. A fouled condenser raises head pressure, not lowers it.