Mark the letter A, B, C or D on your answer sheet to indicate the underlinedpart that needs correction in each of the following questions from 21 to 23.

According to geological discoveries. 4.6-billion-vears life span of our planet is divided into four time intervals called eras.

A. geological discoveries

B. 4.6-billion-vears

C. is

D. into

Dãy nào sau đây đều thuộc loại polime tổng hợp?

A. Tơ capron, tơ nitron, cao su buna.

B. Polistiren, tơ tằm, tơ nilon-6,6.

C. Tơ xenlulozo axetat, cao su buna-S, tơ nilon-6.

D. Tơ visco, tơ olon, tơ nilon-7.