合成红宝石的主要方法（）、（）、（）、（）．
参考答案：焰熔法、水热法、助熔剂法、倒模法/提拉法
Having a trip abroad is certainly good for the old couple, but it remains ______ whether they will enjoy it.[ ]
A. to see
B. to be seen
C. seeing
D. seen
检查发动机风扇皮带张紧度时，用拇指以5-10kg的力按压皮带中间部位，其挠度应为（）左右。
A.3mm
B.10mm
C.20mm
D.30mm
