Code about doctor relation, how to find candidate keys

Question

For my homework I have to develop the sql code for the chart in the picture but for the doctor relation how can I show candidate keys? is there a special way of declaring?

CREATE TABLE "Doctor" (
    "DoctorID"  INTEGER NOT NULL,
    "Superviosr"    TEXT,
    "Name"  TEXT,
    "Phone" INTEGER,
    "SpecialtyNumber"   INTEGER,
    PRIMARY KEY("DoctorID")
);