BCC — Assistant Programmer — Exam: 18.10.2025 (BCC)প্রশ্ন ও সমাধান

Answer: Complex sentence.

• "Though he is poor, he is honest" contains one independent (main) clause — "he is honest" — and one dependent (subordinate) clause — "Though he is poor" — joined by the subordinating conjunction "though".
• A sentence is classified by the number and type of clauses it has:

Type	Structure	Example
Simple	1 independent clause, no subordinate clause	He is honest.
Compound	2+ independent clauses joined by a coordinating conjunction (and, but, or)	He is poor, but he is honest.
Complex	1 independent + 1 (or more) dependent clause, joined by a subordinating conjunction	Though he is poor, he is honest.
Compound-Complex	2+ independent clauses + 1+ dependent clause	Though he is poor, he is honest, and everyone respects him.

• The dependent clause "Though he is poor" cannot stand alone as a complete sentence — it depends on the main clause to make full sense, which is the defining test for a complex sentence.

Exam takeaway: spot subordinating conjunctions (though, although, because, if, since, when, unless, while) — their presence signals a complex sentence.

NAND is called a "universal gate" because any Boolean function — and hence every other basic gate (NOT, AND, OR, NOR, XOR, XNOR) — can be built using NAND gates alone, so a circuit designer never strictly needs any other gate type.

Construction from NAND only:

• NOT gate: tie both inputs of a NAND together → NOT(A) = NAND(A,A)
• AND gate: a NAND followed by a NOT (itself built from NAND) → AND(A,B) = NOT(NAND(A,B))
• OR gate: invert both inputs first, then NAND them (De Morgan's law: A+B = (A′·B′)′) → OR(A,B) = NAND(NOT(A), NOT(B))

graph TD
    subgraph NOT_from_NAND["NOT from NAND"]
    A1((A)) --> G1[NAND]
    A1 --> G1
    G1 --> O1(("A'"))
    end
    subgraph AND_from_NAND["AND from NAND + NOT"]
    A2((A)) --> G2[NAND]
    B2((B)) --> G2
    G2 --> G3[NAND as NOT]
    G2 --> G3
    G3 --> O2(("A AND B"))
    end
    subgraph OR_from_NAND["OR from NAND, De Morgan"]
    A3((A)) --> G4[NAND as NOT]
    A3 --> G4
    B3((B)) --> G5[NAND as NOT]
    B3 --> G5
    G4 --> G6[NAND]
    G5 --> G6
    G6 --> O3(("A OR B"))
    end

Truth table (base NAND):

A	B	Y = (A·B)'
0	0	1
0	1	1
1	0	1
1	1	0

Why this matters (practical reason): chip manufacturers can mass-produce a single gate type (NAND) and wire it into any circuit, simplifying IC design, fabrication, and testing. (NOR is the only other universal gate, by the same De Morgan argument.)

Exam takeaway: NAND (and NOR) is universal because NOT, AND, and OR can each be constructed from NAND gates alone — this is why many digital ICs are built entirely from NAND gates.