单选题Scheduled preventative maintenance visits should be carried out：（）A after two unplanned service calls.B according to manufacturers specifications.C when the device is becoming unreliable.D every sixty days.
题目
单选题
Scheduled preventative maintenance visits should be carried out：（）
A
after two unplanned service calls.
B
according to manufacturers specifications.
C
when the device is becoming unreliable.
D
every sixty days.