Itis（）formarinerstocombinetheSARTwithafloat-freesatelliteEPIRB.
A: essential
B: compulsory
C: optional
D: inadvisable
A: essential
B: compulsory
C: optional
D: inadvisable
举一反三
- Certain courses are compulsory while others are ( ). A: optional B: option C: curriculum D: obligatory
- College students have to take ____________ courses. A: Extracurricular B: Elective C: Compulsory D: Optional
- Business trips are mostly _____________. Meanwhile, the schedule is always full. A: optional B: compulsory C: empty D: friendly
- Inorderto(attain)ourobjective,itis(essential)thatwe(willmake)thebestuseofthelimitedresources(available). A: attain B: essential C: willmake D: available
- ForshipsequippedwithanSES,itis（）and（）fortheoperatortosendadistressalert. A: easy/reliable B: difficult/cheap C: stable/compulsory D: easy/expensive