does primary key in a table can only consist of a single column
The primary key of a table must be unique to identify the record in the table. It's also possible to have a table with a primary key made up of two or more columns. This occurs when a single column can't contain enough unique values. For example, a two column primary key might consist of a FirstName and LastName column. Because primary keys can be made up of more than one column, the PrimaryKey property consists of an array of DataColumn objects.
Primary key constraints perform the same thing as a unique index in the sense that a primary key constraint makes a unique index to enforce the unique values within the data column. A primary key constraint define a column or series of columns that uniquely identify a given row in a table. Defining a primary key on a table is optional and you can only define a single primary key on a table. A primary key constraint can consist of one or many columns (up to 32). Any column that is defined as a primary key column is automatically set with a NOT NULL status.