CHOOSE WORD OR PHRASE - A, B, C OR D - THAT NEEDS CORRECTING

20. - BAI TAP TRAC NGHIEM BAI 910 LOP 11
BAI TAP TRAC NGHIEM BAI 910 LOP 11
20. Choose word or phrase - a, b, c or d - that needs correcting.It was (a) in 1971 when transatlantic (b) supersonic transportation (c) became commercially (d) available.