For Which of the Following Situations Is the Observation Method

For which of the following situations is the observation method of gathering research data best suited?

A) learning how much time people spend selecting bread in the store
B) deciding whether to run an ad for a new cleaning product in Good Housekeeping magazine or in local newspapers
C) finding out what people who spend their vacations at home do with their time
D) estimating the effect of a product modification on sales
E) all of the above