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homomorphism. Pick out the true statements
[NBHM-2006-PhD Screening Test-Algebra]
Let $f : (\Bbb Q,+) \to (\Bbb Q,+)$ be a non-zero homomorphism. Pick out the true statements:
a. $f$ is always one-one.
b. $f$ is always onto.
c. $f$ is always a bijection.
d. $f$ need be neither one-one nor onto.
How can I solve this problem?