Has the process of making synthetic fuels proved cheap or expensive?

B

Quá trình tạo ra nhiên liệu tổng hợp tỏ ra rẻ hay đắt?

→ Nó tỏ ra đắt đỏ.

Thông tin: Experimental work is being done to make synthetic fuels from coal, or coal tars. But to date, that process has proved expensive.