When possible, on UNIX systems, passwords shall not be kept in the /etc/passwd file,
but rather in a shadow password file which can be modified only by root or a program executing on
behalf of root.

The Physical Layer is the layer that is concerned with the signaling of the message and the interface between the sender or receiver and the medium. The physical layer is generally defined by one of the standards bodies and carries a designation that indicates the characteristics of the connection. Among frequently used physical layers standards are EIA-232-D, ITU V.35, and some of the X series (X.21/X.21bis, for example).

What is Relational Algebra:
1.The formal description of how a relational database operates.
2.The mathematics which underpin SQL operations.
A number of operations can be performed in relational algebra to build relations and operate on
the data.
Five operations are primitives (Select, Project, Union, Difference and Product) and the other
operations can be defined in terms of those five. A View is defined from the operations of Join,
Project, and Select.
For the purpose of the exam you must remember the following terms from relational algebra and
their SQL equivalent:
Tuple = Row, Entry
Attribute = Column
Relation or Based relation = Table
See the extract below from the ISC2 book:
Each table, or relation, in the relational model consists of a set of attributes and a set of tuples
(rows) or entries in the table. Attributes correspond to a column in a table. Attributes are unordered
left to right, and thus are referenced by name and not by position. All data values in the relational
model are atomic. Atomic values mean that at every row/column position in every table there is
always exactly one data value and never a set of values. There are no links or pointers connecting
tables; thus, the representation of relationships is contained as data in another table.
A tuple of a table corresponds to a row in the table. Tuples are unordered top to bottom because a
relation is a mathematical set and not a list. Also, because tuples are based on tables that are
mathematical sets, there are no duplicate tuples in a table (sets in mathematics by definition do
not include duplicate elements).
The primary key is an attribute or set of attributes that uniquely identifies a specific instance of an
entity. Each table in a database must have a primary key that is unique to that table. It is a subset
of the candidate key.
Reference used for this question:
Hernandez CISSP, Steven (2012-12-21). Official (ISC)2 Guide to the CISSP CBK, Third Edition
((ISC)2 Press) (Kindle Locations 12262-12269). Auerbach Publications. Kindle Edition.
and
KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of
Computer Security, 2001, John Wiley & Sons, Page 46.
and
http://db.grussell.org/slides/rel%20algebra%201.ppt
NOTE:
SQL offers three classes of operators: select, project, and join.
The select operator serves to shrink the table vertically by eliminating unwanted rows (tuples).
The project operator serves to shrink the table horizontally by removing unwanted columns
(attributes).
And the join operator allows the dynamic linking of two tables that share a common column value.
The join operation is achieved by stating the selection criteria for two tables and equating them
with their common columns.
Most commercial implementations of SQL do not support a project operation, instead projections
are achieved by specifying the columns desired in the output. This is why the Project operator is
not well known as it is fading away from most databases.