Serial activities are those that:

A$.$

Should be performed simultaneously.

B$.$ Klay's shipment of materials hit every red light and traffic jam on the way to the site. Upon arrival, half of the material was judged defective, necessitating a trip back to the supplier, and then the same arduous journey back to the site once the new materials were loaded. Traffic was a little better on the second round but the flat tire and subsequent repair caused as much if not more delay than the first trip. In the final analysis:

A$.$

The project activity of Material Delivery had a much higher variance than anticipated.

B$.$

The project activity of Material Delivery was pretty close to the pessimistic estimate.

C$.$

The project activity of Material Delivery was pretty close to the optimistic estimate.

D$.$

The project activity of Material Delivery was pretty close to the most likely estimate.

Have sequential numbers.

C$.$

Have the same numbers.

D$.$

Flow from one to the next in sequence.