CEFR Level
a colorless, odorless, highly flammable gas that is the lightest and most abundant chemical element in the universe
"Hydrogen is used as a clean fuel in many energy systems."
a wild animal of Africa and Asia that looks like a dog and makes a sound like laughter
"The hyena laughed in the darkness of the night."
Conducive to maintaining health and preventing disease, especially by being clean.
"Maintaining hygienic conditions in hospitals is crucial."
Extremely funny or uncontrollably emotional.
"The comedian's jokes were absolutely hysterical."
