A new eight-kilometre road is under construction that links the port area with the motorway system . It is expected to carry 20,000 trucks and cars a day, which greatly reduces the overcrowded traffic in the center of the city . As part of the project , two four-kilometre road tunnels are being built below the central area of the city , one for traffic to the north and the other for traffic to the south .The two tunnels are about 20 metres below the surface and are 12 metres wide , providing for two lanes of traffic in each direction .
In the upper part of the tunnel two air-conditioning pipes remove the waste gas of trucks and cars and keep the quality of air inside the tunnel . The lighting is at the top of the tunnel , practically at its highest point . There will also be electronic signs at frequent intervals (间隔). They show traffic conditions ahead , and can be seen clearly by drivers . The wall is made up of four main elements , which include a waterproofing covering and , on the inside of the tunnel , a concrete lining (混凝土衬砌).
Each tunnel is roughly round and the lower part of the tunnel is somewhat flat .The surface of the road lies on the base , which is made of concrete and steel The drainage system (排水系统), just below the road surface on one side , removes any extra liquid , particularly water . In the event of fire , the fire main , which is made of steel , pipes water to many fire hydrant stations at regular intervals along the length of the tunnel . The fire main is at the side of the tunnel , and at the level of the road surface . Other systems in the tunnel will include emergency phones . 【題組】73．Which part of the tunnel in the diagram is used to pipe out extra water ?
(A)1. (B)3. (C)5. (D)6.
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