A4. The general term of the sequence $0,-1,0,1,0,-1, \ldots$ is:
(A) $a_{n}=(-1)^{n}$
(B) $a_{n}=1-(-1)^{n}$
(C) $a_{n}=\sin \frac{n \pi}{2}$
(D) $a_{n}=\cos \frac{n \pi}{2}$
(E) None of the above.
A4. The sequence with the general term $a_{n}=(-1)^{n}$ is $-1,1,-1,1, \ldots$, the sequence with the general term $a_{n}=1-(-1)^{n}$ is $2,0,2,0, \ldots$, the sequence with the general term $a_{n}=\sin \frac{n \pi}{2}$ is $1,0,-1,0,1, \ldots$, and the sequence with the general term $a_{n}=\cos \frac{n \pi}{2}$ is $0,-1,0,1,0,-1, \ldots$. Therefore, the correct answer is (D).