A.從拱頂向兩拱腳安裝; B.從兩拱腳向拱頂安裝; C.從L/4處分別向拱頂、拱腳處安裝; D.從兩個拱腳、兩個L/4處分別向拱頂方向安裝
A.20°; B.30°; C.40°; D.50°
A.5d; B.7d; C.8d D.10d