The correct answers are B and D, because the length of the Sprint should be chosen based on the level of uncertainty over the technology to be used and the risk of being disconnected from the stakeholders. The Scrum Guide states that “Sprints have consistent durations throughout a development effort. A new Sprint starts immediately after the conclusion of the previous Sprint. Sprints enable predictability by ensuring inspection and adaptation of progress toward a Sprint Goal at least every calendar month. When a Sprint’s horizon is too long the Sprint Goal may become invalid, complexity may rise, and risk may increase.”