Suppose R₁ (A, B) and R₂ (C, D) are two relation schemas. Let r₁ and r₂ be the corresponding relation instances. B is a foreign key that refers to C in R₂. If data in r₁ and r₂ satisfy referential integrity constraints, which of the following is ALWAYS TRUE?

Consider the following relations A, B and C:
A

ID	Name	Age
12	Arun	60
15	Shreya	24
99	Rohit	11

B

ID	Name	Age
15	Shreya	24
25	Hari	40
98	Rohit	20
99	Rohit	11

C

ID	Phone	Area
10	2200	02
99	2100	01

How many tuples does the result of the following SQL query contain?

SELECT A.Id 
FROM A 
WHERE A.Age > ALL (SELECT B.Age 
                   FROM B 
                   WHERE B.Name = 'Arun')

Consider the following relations A, B and C:
A

ID	Name	Age
12	Arun	60
15	Shreya	24
99	Rohit	11

B

ID	Name	Age
15	Shreya	24
25	Hari	40
98	Rohit	20
99	Rohit	11

C

ID	Phone	Area
10	2200	02
99	2100	01

How many tuples does the result of the following relational algebra expression contain? Assume that the schema of A ∪ B is the same as that of A. $$(A\cup B)\bowtie _{A.Id > 40 \vee C.Id < 15} C$$

Consider the following transactions with data items P and Q initialized to zero:

T1 : read (P) ; 
     read (Q) ; 
     if P = 0 then Q : = Q + 1 ; 
     write (Q). 

T2 : read (Q) ; 
     read (P) 
     if Q = 0 then P : = P + 1 ; 
     write (P).

Any non-serial interleaving of T₁ and T₂ for concurrent execution leads to