uncertain or doubtful
I was dubious about the company's claims so I investigated.
Vocabulary like dubious appears on the Digital SAT in Words-in-Context questions, where you choose the word that fits a passage's meaning and tone. You can also drill this word with vocabulary flashcards.
belief; a belief (or system of beliefs) accepted as authoritative by some group or school
fixed belief; a religious doctrine that is proclaimed as true without proof
rule; be larger in number, quantity, power, or importance
inactive; in a condition of biological rest or suspended animation
depressed; directed downward
harsh; unusually or unreasonably harsh or severe
sediment; sediment that has settled at the bottom of a liquid
impurity; worthless or dangerous material that should be removed