blackboard是可数名词；
blackboard
n.黑板；
复数： blackboards；
例句：
On a blackboard, he drew five concentric circles.
他在黑板上画了5个同心圆。
I turned around and wrote the title on the blackboard
我转过身，在黑板上写下了题目。
He wrote the word 'pride' in huge letters on the blackboard.
他在黑板上写了一个大大的单词pride。
With a stick of chalk he wrote her order on a blackboard.
他用一截粉笔将她的指示写在黑板上。
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