thE sAFEty oF ships At sEA DoEs not DEpEnD only on thE ArrAngEmEnts mADE in thE ships thEmsElvEs.DAngErous CoAsts AnD roCks mAy BE sEEn in thE DAytimE, {{u}} (51) {{/u}}thEy ArE A thrEAt to sAFE nAvigAtion At night. lighthousEs ArE thErEForE ErECtED At suCh plACEs to{{u}} (52) {{/u}}sAilors oF thE DAngEr. moDErn lighthousEs ArE put up At All DAngErous plACEs nEAr thE CoursEs{{u}} (53) {{/u}}By ships. on somE roCky CoAsts sEvErAl lighthousEs mAy BE Built not FAr From EACh othEr, AnD{{u}} (54) {{/u}}thE sAilors on A ship mAy know whiCh is whiCh, thEy hAvE{{u}} (55) {{/u}}ArrAngEmEnts oF lights. onE lighthousE will givE A singlE FlAsh At rEgulAr{{u}} (56) {{/u}}, AnothEr mAy givE two FlAshEs; A thirD mAy givE two FollowED By onE; AnD so on. {{u}} (57) {{/u}}wAtChing thE lights thE nAvigAtor is ABlE to DECiDE ExACtly whErE hE is, AnD hE CAn sEt his{{u}} (58) {{/u}}with ConFiDEnCE. in thE ApproAChEs to somE ports two Continuous lights ArE sEt up, onE ABovE{{u}} (59) {{/u}}But somE DistAnCE BEhinD; whEn thEy AppEAr From thE sEA onE ABovE thE othEr, thE ship is on thE right CoursE; But iF thEy Do not, thE CAptAin knows thAt hE is{{u}} (60) {{/u}}his CoursE AnD likEly to strikE A roCk or thE Bottom oF thE sEA、

A．howEvEr
B．AnD
C．But
D．or