测深线布设的主要因素是测线间隔和测线方向，用单波束测深，其主测深线间隔一般为图上（）。
A.5mm
B.10mm
C.15mm
D.20mm
参考答案：B
科学家推测，原始生命的形成的时间，大约的在地球形成后的多少年（ ）
A．10亿年
B．15亿年
C．20亿年
D．30亿年
The ventilation system of your ship has fire dampers restrained by fusible links. Which statement is TRUE?（）
A.A fusible link will automatically open after a fire is extinguished and reset the damper
B.Fusible links must be replaced at every inspection for certification
C.Fusible links are tested by applying a source of heat to them
D.Fusible links must be replaced if a damper is activated
