Two or more different elements chemically combined

This question lacks some parts, it should read "Two or more different elements chemically combined make up a"

The answer is a COMPOUND

Explanation:

A compound is a type of pure substance that is formed as a result of the chemical combination of two or more different elements. During this combination, the atoms of the constituent elements are chemically joined together by CHEMICAL BONDS.

An example of a compound is CaCO3, which contains the following elements: Calcium, Carbon and Oxygen. In this compound, the atoms of the above listed elements are joined together by chemical bonds to form a COMPOUND.

1. the question asks the most appropriate, which seems to rule out "all of the above." your choice appears to be the correct one.

2. the protocol described is good for identifying correlation, but not cause and effect. for cause/effect, you need to devise a protocol that will assign the treatment (or not) to groups that have essentially the same "demographics" with respect to all the other variables. your choice regarding control group seems appropriate. random assignment of treatment (choice d) may be appropriate also. (i think you can argue that b and d are just different descriptions of the same method.)

3. d will randomize the birds, but may not guarantee there are 10 in each group. e will do both, so seems the better choice.

comment on question 2

based on the problem statement, it seems clear that confident, attentive drivers are more likely to use daytime running lights. so, the protocol might need to be devised in such a way that the lights would be turned on or off without the driver's knowledge. (an attentive driver will figure it out.) assigning a confident, attentive driver to a group required to drive without running lights might mess with his/her confidence and attentiveness. you don't want your experiment to do that. (nor do you want the experiment to subject a driver to greater risk of accident as a consequence of driving without running lights.) experiments involving human safety are always problematic.

i think it's c. if you look search electronic symbol on google and then click on wiki it shows you a