1. since the late 1970's, in the face of a severe loss of marketshare in dozens of industries, manufacturers in the United States .... to improve productivity a

1. since the late 1970's, in the face of a severe loss of marketshare in dozens of industries, manufacturers in the United States .... to improve productivity and therefore enhance their international competitiveness-throughcost-cutting programs.
a. have been trying
b. has been trying
c. had been trying
d. was trying
e. is trying

2. some observers .... the dramatic growth in temporary employment that occurred in the United States during the 1980s to increased participation in the workforce by certain groups.
a. attributed
b. was attributed
c. had been attributing
d. have attributed
e. attribute

3. although genetic mutations in bacteria and viruses can lead to epidemics, some epidemics .... by bacteria and viruses that have undergone no significant genetic change.
a. caused
b. is caused
c. are caused
d. is to be caused
e. causes