​In the sequential negotiation games:

A) ​You can induce a change in the strategy in your opponent only if your threat is believed
B) Any threat can induce a change in strategy in your opponent
C) The best threat is the one that you do not have to carry out
D) ​A and C

In sequential negotiation games, a threat is only effective if it is believed by your opponent. Therefore, inducing a change in strategy in your opponent depends on the credibility of your threat. It is always better to choose a threat that you do not have to carry out, as it avoids potential harm and maintains a positive relationship with the opponent. Hence, the best choice is D, as it includes both options A and C.