With regard to Trigonometric Levelling, which one of the following statements is correct at its simplest applications?

• A. Determination of the elevation of stations is based on the observed vertical angles and the horizontal distances
• B. Determination of the horizontal distances is based on the observed vertical angles
• C. Determination of the vertical angles is based on the observed horizontal distances
• D. Determination of the horizontal distances is based on the observed vertical angles and the measured elevations