139
3.
Why don’t stainless steels rust?
A. because they contain carbon
B. because they contain nickel or chromium
C.
because they extremely hard
4.
What are the most common non-ferrous metals?
A. zinc and tin
B. zinc and lead
C. copper and aluminium
5.
What is a frequent element of metal alloys?
A.
hydrogen
B. oxygen
C. copper
6.
What is the best-known natural polymer?
A. rubber
B. thermoplastics
C. thermosets
7.
What plastics can be heated and moulded only once?
A. thermosets
B. thermoplastics
C. polycarbonate