Which of the following attraction is important for trade of frankincense? （ ） A: Wadi Bani Khalid B: Salalah C: The Al Alam Palace D: Ras al-Jinz

（）只用于梯形图中单个常闭触点与左边触点的串联。 A: AAND B: BANI C: CMRD D: DORI