some one me

the manager of a sawmill wants to find out what percentage of the boards that the mill produced are defective.

which sampling methods are likely to be biased?

choose all answers that are correct.

a.
the manager selects 20 boards at random each day for 5 days and examines them.

b.
the manager sends an email to 20 customers who buy lumber from the mill, asking each how many of the boards they bought the past week were defective. results from the first 5 customers to respond are tallied.

c.
on 1 day, an hour after the sawmill begins production, 30 boards in a row are selected and examined.

d.
every 40th board produced over the course of 1 week is selected and examined.

i think c and d am i correct ?