[单选题]跨越通航河流时,大跨越的相间弧垂允许偏差为( )。
A.300 mm
B.500 mm
C.800 mm
D.1 000 mm

{{B}}The Future of the BBC{{/B}}
As a boy growing up in the 1930s in the Midlands, Norman Painting, the son of a railway-man, listened to a new radio service from the British Broadcasting Corporation. His mother hoped he would get a job as a manager at the mine, but listening to the voices from London talking about world affairs, culture and music gave him other ideas. "The radio opened a door to the world," says Mr. Painting, who went on to Oxford University on a scholarship and became an academic before later working for the BBC’s Radio 4 in its long-running soap, "The Archers".
Mr. Painting’s story helps to explain Britain’s devotion to what it calls "public-service broadcasting", and why the state has been standing by the BBC in the financing issue. The debate had raged for years over whether the BBC should still be publicly financed, especially by a license fee paid by all