A4. A three-term increasing arithmetic sequence with the middle term 10 becomes geometric if this term is reduced by 2. We conclude:
(A) the arithmetic sequence is $3,10,17$
(B) the arithmetic sequence is $4,10,16$
(C) the geometric sequence is $8,8,8$
(D) the geometric sequence is $1,8,64$
(E) it is not possible to obtain a geometric sequence from the arithmetic sequence in the described manner
A4. A three-term increasing arithmetic sequence with the middle term 10 can be written as $10-d, 10$, $10+d$, where $d>0$. The corresponding geometric sequence is $10-d, 8, 10+d$ and it holds that $\frac{8}{10-d}=\frac{10+d}{8}$, from which $d^{2}=36$. Due to the condition $d>0$, we have only the solution $d=6$. The arithmetic sequence is thus $4,10,16$.