题目内容
（请给出正确答案）
[主观题]
The noise-killing system is not yet popular mainly because ______.A．it is still being trie
The noise-killing system is not yet popular mainly because ______.
A．it is still being tried out
B．drivers still have their doubts
C．it increases the cost of car production
D．carmakers are not sure if it is necessary
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